Half-rum could become unhealthy (7)
I believe the answer is:
harmful
'unhealthy' is the definition.
(thesaurus)
'half-rum could become' is the wordplay.
'could become' indicates anagramming the letters.
'half'+'rum'='halfrum'
'halfrum' is an anagram of 'HARMFUL'.
(Other definitions for harmful that I've seen before include "Half rum (anag.)" , "Injurious" , "Dangerous, damaging" , "Causing injury or damage, say poison or malice" , "Noxious" .)