Match about to occupy a long time (5)
I believe the answer is:
agree
'match' is the definition.
(as in two tallies agreeing or matching)
'about to occupy a long time' is the wordplay.
'about' becomes 're' (regarding).
'to occupy' means one lot of letters goes inside another (inserted letters occupy or enter the word).
'a long time' becomes 'age' (an age is a period of time).
're' inserted into 'age' is 'AGREE'.
(Other definitions for agree that I've seen before include "get on" , "confess?" , "'Assent, concur (5)'" , "Arrive at a settlement" , "share the same view" .)