Boot’s fit needing nine regularly (6)
I believe the answer is:
wellie
'boot's' is the definition.
(I've seen this before)
'fit needing nine regularly' is the wordplay.
'fit' becomes 'well' (associated in meaning).
'needing' means one lot of letters go next to another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'nine' are 'ie'.
'well'+'ie'='WELLIE'
(Other definitions for wellie that I've seen before include "footwear" , "Rubber boot" , "Waterproof boot" , "Greek letter" , "In the countryside, it may be green" .)