Boot that is fine at first (6)
I believe the answer is:
wellie
'boot' is the definition.
(I've seen this before)
'that is fine at first' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'fine' becomes 'well' ('well' can be similar in meaning to 'fine'**).
'at first' says to put letters next to each other.
'ie' put after 'well' is 'WELLIE'.
(Other definitions for wellie that I've seen before include "Rubber boot" , "energy" , "Greek letter" , "Waterproof boot" , "In the countryside, it may be green" .)