Happening to occur within seventeen (5)
I believe the answer is:
event
'happening' is the definition.
(I've seen this in another clue)
'within seventeen' is the wordplay.
'within' indicates a hidden word.
'EVENT' is hidden within 'seventeen'.
'to occur' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for event that I've seen before include "Anything which happens" , "Occasion; sports contest" , "Planned public or social occasion" , "400 metres?" , "major incident" .)