Small child stuck in helmet that's narrow (7)
I believe the answer is:
limited
'narrow' is the definition.
(similar in meaning)
'small child stuck in helmet' is the wordplay.
'small child' becomes 'mite' (I've seen this before).
'stuck in' is an insertion indicator.
'helmet' becomes 'lid' (slang for crash helmet).
'mite' put into 'lid' is 'LIMITED'.
'that's' is the link.
(Other definitions for limited that I've seen before include "Finite - type of company" , "Confined within bounds" , "Restricted in extent" , "Circumscribed, restricted" , "Narrow" .)