Small child learner one had met with crash helmet (7)
I believe the answer is:
skidlid
'crash helmet' is the definition.
(I've seen this before)
'small child learner one had met' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'child' becomes 'kid' (synonyms).
'learner' becomes 'l'.
'one had' becomes 'id' ('I'd' can mean 'one had').
'met' is a charade indicator (letters next to each other).
's'+'kid'+'l'+'id'='SKIDLID'
'with' is the link.
(Other definitions for skidlid that I've seen before include "Crash helmet (slang)" , "protection" , "Crash-helmet" .)