Courageous bloke grabbing first of thieves, sneaky (5)
I believe the answer is:
gutsy
'courageous' is the definition.
(synonyms)
'bloke grabbing first of thieves sneaky' is the wordplay.
'bloke' becomes 'guy' (I've seen this in another clue).
'grabbing' means one lot of letters goes inside another.
'first of' indicates taking the first letters.
The initial letters of 'thieves sneaky' is 'ts'.
'guy' going around 'ts' is 'GUTSY'.
(Other definitions for gutsy that I've seen before include "Courageous, spirited" , "Spirited; greedy" , "Plucky" , "Sort of (non-standard)" , "Bold" .)