It covers pot belly fine, ultimately coat's for going out (3,4)
I believe the answer is:
'it covers pot' is the definition.
(item covering a tea pot)
'belly fine ultimately coat's for going out' is the wordplay.
'ultimately' says to take the final letters.
'for going out' indicates an anagram (letters go out of place).
The last letters of 'belly fine' are 'ye'.
'yecoats' anagrammed gives 'TEA-COSY'.
(Other definitions for tea cosy that I've seen before include "put on for warmth" , "It keeps the beverage hot" , "Knitted item" , "leaves in a hot state in this?" , "Pot cover" .)