Referring to D. J. Bernstein's paper Bernstein-Curve25519 on Curve25519, the group $\{\infty\} \cup (E(F_{p^2}) \cap (F_p \times F_p))$, where $p$ is the prime $2^{255}-19$ and $E$ is the elliptic curve $y^2 = x^3 + 486662x^2 + x$, has size $n = 8p_1$, where $p_1$ is the large prime $2^{252} + 27742317777372353535851937790883648493$.
In the same paper, he suggests clearing the 3 least significant bits of the 255-bit secret scalar $k \in [1, n-1]$ (for elliptic curve scalar multiplication over Curve25519), that is, making $k$ a multiple of 8, to prevent small sub-group attacks. Note that this also reduces the effective security level of the curve to $\approx 2^{252}$.
Instead, is it equally safe to choose the scalar $k$ from $[1, p_1-1]$, that is, operate over the prime sub-group of Curve25519? This does not change any of the curve operations, only the scalar is chosen differently. Also, small sub-group attacks are not possible since $k < p_1$ in this case, and the effective security level is again reduced to $\approx 2^{252}$.