Boot that is following fit (6)
I believe the answer is:
wellie
'boot' is the definition.
(I've seen this before)
'that is following fit' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'following' means one lot of letters go next to another.
'fit' becomes 'well' ('well' is associated in meaning with 'fit').
'ie' put after 'well' is 'WELLIE'.
(Other definitions for wellie that I've seen before include "energy" , "footwear" , "Waterproof boot" , "Greek letter" , "In the countryside, it may be green" .)