I left after endless waiting around first (7)
I believe the answer is:
initial
'first' is the definition.
('initial' can be similar in meaning to 'first')
'i left after endless waiting around' is the wordplay.
'left' becomes 'l' (common abbreviation**).
'after' says to put letters next to each other.
'endless' says to take the centre.
'around' says the letters should be written backwards.
The middle of 'waiting' is 'aitin'.
'l'+'aitin'='laitin'
'laitin' in reverse letter order is 'nitial'.
'i'+'nitial'='INITIAL'
(Other definitions for initial that I've seen before include "Occurring at the start" , "Approve" , "Start of name" , "Word's first letter" , "Letter - opening" .)