If we assume that we are working in cartesian coor. system, we can write;
\hat{e}_i \epsilon_{ijk} \nabla_j (\nabla a)_k = \hat{e}_i \epsilon_{ijk} \partial_j \partial_k a
here \nabla_i = \partial_i
then let's write this sum term by term:
\sum_{ijk} \hat{e}_i \epsilon_{ijk} \partial_j...