Stern on vessel in difficulty (8)
I believe the answer is:
hardship
'difficulty' is the definition.
(hardship is a kind of difficulty)
'stern on vessel' is the wordplay.
'stern' becomes 'hard' (I've seen this in another clue).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'vessel' becomes 'ship' (ship is a kind of vessel).
'hard'+'ship'='HARDSHIP'
'in' is the link.
(Other definitions for hardship that I've seen before include "Adversity, misfortune" , "Severe discomfort or deprivation" , "distress" , "Need" , "Severe privation or suffering" .)