A Firth the army may go round for nothing (5)
I believe the answer is:
'a firth' is the definition.
(I've seen this before)
'army may go round for nothing' is the wordplay.
I cannot really see how this works, but
'go' could be 'ray' (raying is a kind of going) and 'ray' is found in the answer.
'round' could be 'o' (round can mean a round object) and 'o' is present in the answer.
A single letter 'm' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for moray that I've seen before include "Predator eel" , "is fishy" , "Firth" , "One fish" , "seen snaking near the coast?" .)