Goalpost periodically breaks (4)
I believe the answer is:
gaps
'breaks' is the definition.
('gap' can be a synonym of 'break')
'goalpost periodically' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternating letters of 'goalpost' are 'GAPS'.
(Other definitions for gaps that I've seen before include "Breaks, spaces" , "Breaks or openings as in hedge or fence" .)