Gathering pace finally, force trial to go ahead (8)
I believe the answer is:
jamboree
'gathering' is the definition.
(I've seen this before)
'pace finally force trial to go ahead' is the wordplay.
'finally' says to take the final letters.
'force' becomes 'jam' (jamming is a kind of forcing**).
'trial' becomes 'bore'.
'to go ahead' is a charade indicator (letters next to each other).
The last letter of 'pace' is 'e'.
'jam'+'bore'='jambore'
'e' put after 'jambore' is 'JAMBOREE'.
(Other definitions for jamboree that I've seen before include "Scout 1 across" , "Spree" , "Large and lively party" , "Large scout rally" , "Boy Scouts' international gathering" .)