Exceptional leave I declined joining the Sappers (7)
I believe the answer is:
extreme
'exceptional' is the definition.
('extreme' can be a synonym of 'exceptional')
'i declined joining the sappers' is the wordplay.
I cannot really see how this works, but
'i' could be 'me' and 'me' is present in the answer.
'sappers' could be 're' (Royal Engineers) and 're' is present in the answer.
This explanation may well be incorrect...
'leave' acts as a link.
(Other definitions for extreme that I've seen before include "Farthest - to the highest degree" , "Acute, maximum" , "Meet Rex (anag.)" , "Last" , "Greatest or smallest" .)