A comparison which makes one break into a grin (6)
I believe the answer is:
simile
'a comparison' is the definition.
(simile is a kind of comparison)
'one break into a grin' is the wordplay.
'one' becomes 'i' (Roman numeral).
'break into' means one lot of letters goes inside another.
'a grin' becomes 'smile' ('smile' can be a synonym of 'grin').
'i' placed into 'smile' is 'SIMILE'.
'which makes' is the link.
(Other definitions for simile that I've seen before include "Most common figure of speech, one of comparison" , "Basic figure of speech, makes comparison" , "as silly as a loon, say" , "as easy as pie, perhaps" , "as happy as Larry?" .)