Heavy stomach husband fills up (4)
I believe the answer is:
thug
'heavy' is the definition.
(I've seen this in another clue)
'stomach husband fills up' is the wordplay.
'stomach' becomes 'gut' (I've seen this in another clue).
'husband' becomes 'h' (abbreviation).
'fills' means one lot of letters goes inside another (one set of letters goes inside another).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'gut' placed around 'h' is 'guht'.
'guht' reversed gives 'THUG'.
(Other definitions for thug that I've seen before include "Brutal criminal" , "Gangster" , "Violent criminal" , "brute" , "undesirable character" .)