Sooner than be at the front (6)
I believe the answer is:
before
'sooner than' is the definition.
(I've seen this in another clue)
'be at the front' is the wordplay.
'at' means one lot of letters go next to another.
'the front' becomes 'fore' (fore is a kind of front).
'be'+'fore'='BEFORE'
(Other definitions for before that I've seen before include "Defy, withstand" , "On a previous occasion" , "In sight of" , "During the period preceding" , "In front - earlier" .)