Grand in small bet for brag (7)
I believe the answer is:
swagger
'for brag' is the definition.
(thesaurus)
'grand in small bet' is the wordplay.
'grand' becomes 'g' (G is short for grand as in a thousand).
'in' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bet' becomes 'wager' (synonyms).
's'+'wager'='swager'
'g' going within 'swager' is 'SWAGGER'.
(Other definitions for swagger that I've seen before include "confident gait" , "Walk self-importantly, strut" , "Blustering arrogance" , "Walk with an air of self-importance" , "Move confidently" .)