Ace forward, certain to delay header, finds gaps (9)
I believe the answer is:
apertures
'gaps' is the definition.
(aperture is a kind of gap)
'ace forward certain' is the wordplay.
'ace' becomes 'a'.
'forward' becomes 'pert' (I've seen this in other clues).
'certain' becomes 'ures' (I can't justify this - if you can you should believe this answer much more).
'a'+'pert'+'ures'='APERTURES'
'to delay header finds' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Another definition for apertures that I've seen is " 'Holes, openings (9)'".)