Jack's said to express gratitude (3)
I believe the answer is:
tar
'jack's' is the definition.
(both are informal terms for sailor)
'to express gratitude' is the wordplay.
'to express' shows a homophone (sound like) (I've seen 'expressed' mean this).
'gratitude' becomes 'ta' (I've seen this in other clues).
'ta' sounds like 'TAR'.
'said' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for tar that I've seen before include "Tobacco residue; sailor" , "Cigarette substance" , "Harmful tobacco residue" , "Informal sailor or coal distillation" , "Naval man" .)