Small child in top is constrained (7)
I believe the answer is:
limited
'is constrained' is the definition.
(I've seen this before)
'small child in top' is the wordplay.
'small child' becomes 'mite' (informal term).
'in' means one lot of letters goes inside another.
'top' becomes 'lid' (lid is a kind of top).
'mite' put inside 'lid' is 'LIMITED'.
(Other definitions for limited that I've seen before include "was restricting" , "Finite - type of company" , "Restricted in extent" , "short of talent" , "Constrained" .)