By start of March, engaging in obligations (5)
I believe the answer is:
times
'by' is the definition.
('times' can mean multiplied by)
'start of march engaging in obligations' is the wordplay.
'start of' indicates taking the first letters.
'engaging in' is an insertion indicator (I've seen this in other clues).
'obligations' becomes 'ties' (I've seen this before).
The first letter of 'march' is 'm'.
'm' put inside 'ties' is 'TIMES'.
(Other definitions for times that I've seen before include "''For the ..., they are a'changin'''(Bob Dylan)" , "Occasions; multiplied by" , "See 10" , "Square in New York" , "measures the length of something" .)