Venture to say it could go on forever (5)
I believe the answer is:
foray
'venture' is the definition.
(I've seen this in another clue)
'to say it could go on forever' is the wordplay.
I cannot quite understand how this works, but
'to' could be 'for' and 'for' is found within the answer.
'forever' could be 'ay' (historic term for 'forever') and 'ay' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for foray that I've seen before include "Raid or incursion" , "Raid for plunder" , "Sortie" , "Brief attack" , "Raid into enemy territory" .)