Gangster is ahead of customs every time (6)
I believe the answer is:
always
'every time' is the definition.
(I've seen this in another clue)
'gangster is ahead of customs' is the wordplay.
'gangster' becomes 'al' (Al Capone).
'is ahead of' says to put letters next to each other (some letters go in front of others).
'customs' becomes 'ways' (I've seen this before).
'al'+'ways'='ALWAYS'
(Other definitions for always that I've seen before include "For good" , "In perpetuity" , "Every time" , "Forever, without interruption" , "Habitually" .)