Prior to Quins trials, dropped now and then (5)
I believe the answer is:
until
'prior to' is the definition.
(I've seen this in another clue)
'quins trials dropped now and then' is the wordplay.
'dropped now and then' indicates alternate letters.
'quins'+'trials'='quinstrials'
The alternate letters of 'quinstrials' are 'UNTIL'.
(Other definitions for until that I've seen before include "As far as" , "Up to (a point in time)" , "As late as" , "Unlit (anag.)" , "Up to that moment" .)