Which unit of power did you say? (4)
I believe the answer is:
what
'which' is the definition.
(I've seen this in other clues)
'unit of power did you say?' is the wordplay.
'unit of power' becomes 'watt' (I've seen this before).
'did you say?' shows a homophone (sound like).
'watt' sounds like 'WHAT'.
(Other definitions for what that I've seen before include "Eh?" , "That which - come again!" , "I beg your pardon" , "Which thing?" , "Word used in a question" .)