Head in charge? It's debatable (5)
I believe the answer is:
topic
'it's debatable' is the definition.
(I've seen this in another clue)
'head in charge?' is the wordplay.
'head' becomes 'top' (synonyms).
'in charge?' becomes 'ic' (abbreviation for 'in charge').
'top'+'ic'='TOPIC'
(Other definitions for topic that I've seen before include "might this be hot?" , "Content of argument" , "Subject (of speech, etc)" , "Theme of discourse" , "That may be debatable" .)