Margin accepted by one doing trial period (9)
I believe the answer is:
trimester
'period' is the definition.
(trimester is a kind of period)
'margin accepted by one doing trial' is the wordplay.
'margin' becomes 'rim'.
'accepted by' means one lot of letters goes inside another.
'one doing trial' becomes 'tester' (someone performing a test or trial).
'rim' going within 'tester' is 'TRIMESTER'.
(Other definitions for trimester that I've seen before include "academic session" , "Period of three months" , "part of college year" , "American term" , "US academic term" .)