It's observed to dishearten seamen (4)
I believe the answer is:
seen
'it's observed' is the definition.
(synonyms)
'dishearten seamen' is the wordplay.
'dishearten' says to hollow out the word (remove centre letters).
'seamen' with its middle taken out is 'SEEN'.
'to' is the link.
(Other definitions for seen that I've seen before include "Distinguished" , "Observed or percieved" , "Met - understood" , "Espied" , "Noticed" .)