Devil getting on with mostly agreeable Kelvin (3,4)
I believe the answer is:
old nick
'devil' is the definition.
('Old Nick' can be a synonym of 'devil')
'getting on with mostly agreeable kelvin' is the wordplay.
'getting on' becomes 'old' ('getting on a bit' means 'old').
'with' is a charade indicator (letters next to each other).
'mostly' means to remove the last letter.
'agreeable' becomes 'nice' (I've seen this before).
'kelvin' becomes 'k' ('K' can be a synonym of 'kelvin').
'nice' with its final letter taken off is 'nic'.
'old'+'nic'+'k'='OLD NICK'
(Other definitions for old nick that I've seen before include "Newgate or Marshalsea?" , "The Evil One" , "Fleet, for example" , "alternative to deep blue sea" , "reopened wound?" .)