Adult, small below, in pieces (7)
I believe the answer is:
asunder
'in pieces' is the definition.
(I've seen this before)
'adult small below' is the wordplay.
'adult' becomes 'a'.
'small' becomes 's' (abbreviation - e.g. clothes size).
'below' becomes 'under' ('under' can be a synonym of 'below').
'a'+'s'+'under'='ASUNDER'
(Other definitions for asunder that I've seen before include "(Break) apart" , "bits?" , "Into parts or pieces" , "(Rent) apart" , "(Torn) into parts" .)