Spin perhaps is mostly nonsense (5)
I believe the answer is:
drive
'spin' is the definition.
(I've seen this before)
'is mostly nonsense' is the wordplay.
'is mostly' means to remove the last letter (most but not all of the word).
'nonsense' becomes 'drivel' (drivel can mean nonsensical speech or writing).
'drivel' with its final letter removed is 'DRIVE'.
'perhaps' acts as a link.
(Other definitions for drive that I've seen before include "Use Aston Martin, say" , "Road off main highway" , "Energy < tax" , "Avenue" , "spin" .)