Shallow sort involved in leak (4-4)
I believe the answer is:
skin-deep
'shallow' is the definition.
(I've seen this before)
'sort involved in leak' is the wordplay.
'sort' becomes 'kind' (synonyms).
'involved in' is an insertion indicator.
'leak' becomes 'seep' (I've seen this in another clue).
'kind' placed within 'seep' is 'SKIN-DEEP'.
(Other definitions for skin-deep that I've seen before include "Superficial, beauty perhaps" , "Develop" , "Superficial, barely penetrating" , "Superficial (like beauty?)" , "Stringed instrument" .)