Be forced to accept the number will decrease drastically (5)
I believe the answer is:
halve
'be forced' is the definition.
I can't judge whether this defines the answer.
'to accept the number will decrease drastically' is the wordplay.
'to accept' indicates putting letters inside.
'the number' becomes 'v' (V is an example).
'will decrease drastically' becomes 'hale' (I can't explain this - if you can you should believe this answer much more).
'v' inserted inside 'hale' is 'HALVE'.
Can you help me to learn more?
(Other definitions for halve that I've seen before include "Divide into two equal pieces" , "Cut down substantially" , "make 10 5 say?" , "Divide fifty-fifty" , "cut by fifty percent" .)