The main worry for the old sailor (6)
I believe the answer is:
seadog
'the old sailor' is the definition.
The definition suggests a singular noun which matches the answer.
'the main worry' is the wordplay.
'the main' becomes 'sea' (the main is an old name for the sea).
'worry' becomes 'dog' (I've seen this before).
'sea'+'dog'='SEADOG'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for seadog that I've seen before include "Mate, perhaps" , "Rating" .)