Rather kind old fellow (4,2)
I believe the answer is:
sort of
'rather' is the definition.
(thesaurus)
'kind old fellow' is the wordplay.
'kind' becomes 'sort' ('sort' can be a synonym of 'kind'**).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'sort'+'o'+'f'='SORT OF'
(Other definitions for sort of that I've seen before include "Not exactly" , "in a way" , "Not quite, but almost" , "Somewhat" , "Possibly, to an extent" .)