Boorish type, mostly a hooligan at heart
I believe the answer is:
yahoo
'boorish type' is the definition.
(I've seen this in another clue)
'mostly a hooligan at heart' is the wordplay.
'at heart' says the answer is hidden in the clue (letters hidden in the heart of the clue).
'YAHOO' can be found hidden inside 'mostly a hooligan'.
(Other definitions for yahoo that I've seen before include "Brutish animal species from Gulliver's Travels" , "Ugly swipe at delivery" , "Brutish person created by Dean Swift" , "Swift chap" , "crude fellow" .)