Five go astray, cutting round a French swamp (7)
I believe the answer is:
overrun
'swamp' is the definition.
(I've seen this before)
'five go astray cutting round a french' is the wordplay.
'five' becomes 'V' (Roman numeral).
'go astray' becomes 'err' (to err is to make an error or go the wrong way).
'cutting' is an insertion indicator (some letters cut their way into a word).
'round' becomes 'o' (round can mean a round object).
'a french' becomes 'un' ('a' in French).
'v'+'err'='verr'
'o'+'un'='oun'
'verr' going into 'oun' is 'OVERRUN'.
(Other definitions for overrun that I've seen before include "Exceed (estimated cost)" , "last longer than planned" , "Exceed (the expected time or cost)" , "Heavily defeat" , "Occupy (a place) in large numbers" .)