That chap apt to breathe on one a lot?
I believe the answer is:
him
'that chap' is the definition.
(I've seen this in another clue)
'apt to breathe on one a lot?' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'lot?' could be 'm' and 'm' is present in the answer.
A single letter 'h' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for him that I've seen before include "Proper character" , "That man" , "Objective male pronoun" .)