Inferred they'd express gratitude for brief mention (5)
I believe the answer is:
tacit
'inferred' is the definition.
(I've seen this before)
'gratitude for brief mention' is the wordplay.
'gratitude' becomes 'ta' (I've seen this in other clues).
'for' means one lot of letters go next to another.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'mention' becomes 'cite' (to cite is to mention or quote).
'cite' with its last letter taken away is 'cit'.
'ta'+'cit'='TACIT'
'they'd express' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for tacit that I've seen before include "Understood without being directly expressed" , "Unspoken but implied" , "Implied, unstated" , "Aircraft engine casing" , "Implicit, unspoken" .)