Jack's said to express gratitude (3)
I believe the answer is:
tar
'jack's' is the definition.
(both are informal terms for sailor)
'said to express gratitude' is the wordplay.
'said' becomes 't' (I've seen this in other clues).
'to' says to put letters next to each other.
'express gratitude' becomes 'ar' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
't'+'ar'='TAR'
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" .)