Concentrate because in manuscript
I believe the answer is:
mass
'concentrate' is the definition.
(I've seen this in another clue)
'because in manuscript' is the wordplay.
'because' becomes 'as' (I've seen this in other clues).
'in' indicates putting letters inside.
'manuscript' becomes 'ms' (abbreviation for manuscript).
'as' inserted inside 'ms' is 'MASS'.
(Other definitions for mass that I've seen before include "Large amount, body" , "Quantity; religious service" , "Heap; religious service" , "seasonal midnight service?" , "Throng" .)