A letter I had, making an offer (3)
I believe the answer is:
bid
'an offer' is the definition.
(bid is a kind of offer)
'a letter i had' is the wordplay.
'a letter' becomes 'b' ().
'i had' becomes 'id' (common contraction).
'b'+'id'='BID'
'making' acts as a link.
(Other definitions for bid that I've seen before include "Order - offer" , "Offer; command" , "Make offer for 20 Down" , "Offer (for); demand" , "Take part in auction" .)