Kind endlessly backing official reason (5)
I believe the answer is:
infer
'reason' is the definition.
(inferring is a kind of reasoning)
'kind endlessly backing official' is the wordplay.
'endlessly' says to take the centre (the ends of the words are removed).
'backing' says the letters should be written in reverse.
'official' becomes 'ref' (match official in football, say).
The central letters of 'kind' are 'in'.
'ref' backwards is 'fer'.
'in'+'fer'='INFER'
(Other definitions for infer that I've seen before include "Extrapolate, conclude" , "come to a conclusion" , "'Deduce, conclude (5)'" , "Conclude or judge from facts" , "Work out" .)