With no end in sight, ask the man why breather isn't okay? (6)
I believe the answer is:
asthma
'why breather isn't okay?' is the definition.
(breathing problem)
'with no end in sight ask the man' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'end' could be 'ash' (I have seen 'End of fire' mean 'ash' so perhaps 'end' could also mean 'ash') and 'ash' is present in the remaining letters.
'man' could be 'm' and 'm' is present in the answer.
A single letter 'a' 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).
(Other definitions for asthma that I've seen before include "Lung condition" , "which leaves one breathless" , "Breathing complaint" , "Common breathing ailment" , "Bronchial allergy" .)