I had to stand in line -- it's neater (6)
I believe the answer is:
tidier
'neater' is the definition.
('tidy' can be a synonym of 'neat')
'i had to stand in line' is the wordplay.
'i had' becomes 'id' (I'd).
'to stand' is a reversal indicator (I've seen 'standing' mean this).
'in' is an insertion indicator.
'line' becomes 'tier' (tier is a kind of line**).
'id' in reverse letter order is 'di'.
'di' going into 'tier' is 'TIDIER'.
'it's' acts as a link.
(Other definitions for tidier that I've seen before include "'Neater, in better order (6)'" , "More presentable" , "More trim" , "not so disorderly" , "More neat and orderly" .)