Discharge when ready, oddly forgotten sidearm (7)
I believe the answer is:
cashier
'discharge' is the definition.
(cashiering is a kind of discharging)
'ready oddly forgotten sidearm' is the wordplay.
'ready' becomes 'cash' (ready can informally mean cash).
'oddly forgotten' indicates alternate letters (odd-numbered letters get removed).
The alternating letters of 'sidearm' are 'ier'.
'cash'+'ier'='CASHIER'
'when' is the link.
(Other definitions for cashier that I've seen before include "one taking money" , "money dispenser" , "Till operator" , "till tender?" , "Sack" .)