Boast about habit (4)
I believe the answer is:
brag
'boast' is the definition.
(brag is a kind of boast)
'about habit' is the wordplay.
'about' shows that the letters should be reversed in order.
'habit' becomes 'garb' (I've seen this before).
'garb' written backwards gives 'BRAG'.
(Other definitions for brag that I've seen before include "boast; form of poker" , "Boast ? card-game" , "Show off, crow" , "Boast about garb" , "Boast, crow" .)