Snails occasionally going through plant (6)
I believe the answer is:
salvia
'plant' is the definition.
(salvia is a kind of plant)
'snails occasionally going through' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'going through' becomes 'via' (via means through).
The alternating letters of 'snails' are 'sal'.
'sal'+'via'='SALVIA'
(Other definitions for salvia that I've seen before include "Sage, or bedding plant" , "Plant of genus that includes sage" , "This herb" , "Herbaceous plant - shrub" , "Sage, for example" .)