Complimentary cover ultimately incorporated into cost (4)
I believe the answer is:
free
'complimentary' is the definition.
(synonyms)
'cover ultimately incorporated into cost' is the wordplay.
'ultimately' suggests the final letters.
'incorporated into' is an insertion indicator.
'cost' becomes 'fee' (fee is a kind of cost).
The final letter of 'cover' is 'r'.
'r' placed inside 'fee' is 'FREE'.
(Other definitions for free that I've seen before include "nought" , "Gratis, for nothing" , "Lavish" , "On the house" , "Reef (anag.)" .)