In contrast, heartache seems macho (5)
I believe the answer is:
butch
'macho' is the definition.
(I know that macho can be written as butch)
'in contrast heartache' is the wordplay.
'in contrast' becomes 'but' (but on the other hand...).
'heartache' becomes 'ch' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'but'+'ch'='BUTCH'
'seems' acts as a link.
Can you help me to learn more?
(Other definitions for butch that I've seen before include "Acting as a male (colloq.)" , "Tough" , "Markedly masculine" , "Macho" , "Aggressively masculine" .)