Child’s foot too small to fit inside restraint (7)
I believe the answer is:
tootsie
'child's foot' is the definition.
(childish term for a foot)
'too small to fit inside restraint' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'to fit' indicates anagramming the letters (letters must 'fit' together differently).
'inside' indicates putting letters inside.
'restraint' becomes 'tie' (I've seen this before).
'too'+'s'='toos'
'toos' is an anagram of 'oots'.
'oots' placed inside 'tie' is 'TOOTSIE'.
(Other definitions for tootsie that I've seen before include "pretty girl" , "Foot (jocular)" , "Foot or toe - I set, too (anag)" , "What's furthest from a child's mind" , "digit" .)