I've sent back dip that's plain (7)
I believe the answer is:
evident
'that's plain' is the definition.
(synonyms)
'i've sent back dip' is the wordplay.
'sent back' says the letters should be written backwards.
'dip' becomes 'dent'.
'ive' backwards is 'evi'.
'evi'+'dent'='EVIDENT'
(Other definitions for evident that I've seen before include "Quite obvious, manifest" , "Indisputable" , "Incontrovertible" , "Conspicuous" , "clearly visible" .)