It's a pain when lookalike gets half-cut (6)
I believe the answer is:
twinge
'it's a pain' is the definition.
(twinge is a kind of pain)
'lookalike gets half-cut' is the wordplay.
'lookalike' becomes 'twin' (twin can mean a lookalike).
'half cut' indicates taking half.
'gets' with half the letters taken is 'ge'.
'twin'+'ge'='TWINGE'
'when' acts as a link.
(Other definitions for twinge that I've seen before include "Brief sudden pain" , "A slight stab of pain" , "Slight pang, maybe of conscience" , "Sudden darting pain" , "Brief sharp pain" .)