Popular and match fit, beyond dispute (13)
I believe the answer is:
incontestable
'beyond dispute' is the definition.
(I've seen this before)
'popular and match fit' is the wordplay.
'popular' becomes 'in'.
'and' means one lot of letters go next to another.
'match' becomes 'contest' (I've seen this before**).
'fit' becomes 'able' ('able' can be similar in meaning to 'fit').
'in'+'contest'+'able'='INCONTESTABLE'
(Other definitions for incontestable that I've seen before include "Beyond dispute" , "sure" , "Without doubt" .)