Nowadays suitable to alter (5)
I believe the answer is:
adapt
'to alter' is the definition.
(adapting is a kind of altering)
'nowadays suitable' is the wordplay.
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'suitable' becomes 'apt' (apt means appropriate or fitting).
'ad'+'apt'='ADAPT'
(Other definitions for adapt that I've seen before include "rewrite for broadcasting" , "Accommodate" , "match" , "Modify; acclimatise" , "Put to particular use" .)