Diminished with return thanks in bed (5)
I believe the answer is:
bated
'diminished' is the definition.
(thesaurus)
'return thanks in bed' is the wordplay.
'return' says the letters should be written backwards.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'in' is an insertion indicator.
'ta' in reverse letter order is 'at'.
'at' put inside 'bed' is 'BATED'.
'with' acts as a link.
(Other definitions for bated that I've seen before include "With - breath means in great suspense" , "Moderated, the breath in anticipation, say" , "Reduced (referring to breath)" .)