Vital for credit union agents to be online (7)
I believe the answer is:
crucial
'vital' is the definition.
(I know that vital can be written as crucial)
'credit union agents to be online' is the wordplay.
'credit' becomes 'cr'.
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'agents' becomes 'CIA' (Central Intelligence Agency).
'to be online' becomes 'l' (I am not sure about this - if you are sure you should believe this answer much more).
'cr'+'u'+'cia'+'l'='CRUCIAL'
'for' acts as a link.
(Other definitions for crucial that I've seen before include "Of vital importance" , "Of paramount importance" , "Pivotal" , "Of great importance" , "'Critical, vital (7)'" .)