Power cut? That's a relief (4)
I believe the answer is:
phew
'that's a relief' is the definition.
'power cut?' is the wordplay.
'power' becomes 'p'.
'cut?' becomes 'hew' (I've seen this before**).
'p'+'hew'='PHEW'
(Other definitions for phew that I've seen before include "Gosh, that's a relief!" , "Exclamation of relief" , "Gosh, I'm sweltering!" , "I'm overheating?" , "thank goodness!" .)