Grasp every so often there’s an opening (3)
I believe the answer is:
gap
'there's an opening' is the definition.
(gap is a kind of opening)
'grasp every so often' is the wordplay.
'every so often' means one should take alternating letters.
The alternate letters of 'grasp' are 'GAP'.
(Other definitions for gap that I've seen before include "Pause" , "Break in continuity" , "Disparity" , "Empty space between things" , "Hiatus" .)