Shift at sea with Coastguard — be in good shape (3,2,1,8,3)
I believe the answer is:
fit as a butchers dog
Here is my best explanation:
'shift at sea with coastguard be in good shape' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'shift at sea with coastguard be' is the wordplay.
'at sea with' is an anagram indicator.
'shift' mixed up with 'coastguardbe' gives 'FIT AS A BUTCHERS DOG'.