Small, pert child holding small rock (6)
I believe the answer is:
schist
'rock' is the definition.
(schist is a kind of rock)
'small pert child holding small' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pert child' becomes 'chit'.
'holding' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'chit' going around 's' is 'chist'.
's'+'chist'='SCHIST'
(Other definitions for schist that I've seen before include "Coarse-grained rock" , "layered rock" , "Form of rock" .)