Sweet letter for us, tucked inside a card (7)
I believe the answer is:
custard
'sweet' is the definition.
(I've seen this before)
'us tucked inside a card' is the wordplay.
'tucked' becomes 't' (this could be a standard abbreviation I've not previously seen).
'inside' is an insertion indicator.
'us'+'t'='ust'
'ust' inserted within 'card' is 'CUSTARD'.
'letter for' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for custard that I've seen before include "Egg/milk bake" , "Type of apple" , "Sweet sauce made with milk and eggs" , "Yellow sauce" , "Pudding sauce" .)