Left in pain, that's clear (5)
I believe the answer is:
plain
'clear' is the definition.
(synonyms)
'left in pain' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'l' inserted within 'pain' is 'PLAIN'.
'that's' acts as a link.
(Other definitions for plain that I've seen before include "ugly" , "Unembellished" , "Simple, unadorned" , "no oil painting features" , "Unflavoured" .)