Narrow and small child in helmet (7)
I believe the answer is:
limited
'narrow' is the definition.
(similar in meaning)
'small child in helmet' is the wordplay.
'small child' becomes 'mite' (informal term).
'in' indicates putting letters inside.
'helmet' becomes 'lid' (slang for crash helmet).
'mite' going into 'lid' is 'LIMITED'.
'and' acts as a link.
(Other definitions for limited that I've seen before include "Bound" , "Confined within bounds" , "Restricted in extent" , "Restricted, finite" , "Bounded, circumscribed" .)