Decline of a gent, oddly in condition (8)
I believe the answer is:
stagnate
'decline' is the definition.
(to stagnate is to stop developing or decline)
'a gent oddly in condition' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'in' is an insertion indicator.
'condition' becomes 'state' (the state something is in).
The alternate letters of 'gent' are 'gn'.
'a'+'gn'='agn'
'agn' put inside 'state' is 'STAGNATE'.
'of' acts as a link.
(Other definitions for stagnate that I've seen before include "Do nothing" , "decline" , "Become still and stale, pond or people (8)" , "Cease activity" , "Become still or stale through inactivity" .)