Choose against old, time to get energetic primarily (4)
I believe the answer is:
vote
'choose' is the definition.
(voting is a kind of choosing)
'against old time to get energetic primarily' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'time' becomes 't' (abbreviation).
'to get' means one lot of letters go next to another.
'primarily' indicates taking the first letters.
The first letter of 'energetic' is 'e'.
'v'+'o'+'t'+'e'='VOTE'
(Other definitions for vote that I've seen before include "Register one's choice" , "Ballot, exercise of franchise" , "Formal indication of opinion in an election" , "Mark an 'X'" , "Elector's choice" .)