Member's problem when drink on the counter swiped by Trojan king (8)
I believe the answer is:
priapism
'member's problem' is the definition.
(problem of constant erections - penis is a member)
'drink on the counter swiped by trojan king' is the wordplay.
'drink' becomes 'sip' (sipping is a kind of drinking).
'on the counter' is a reversal indicator (counter can mean in the opposite direction).
'swiped by' means one lot of letters goes inside another.
'trojan king' becomes 'priam' (king of Troy in Greek mythology).
'sip' written backwards gives 'pis'.
'pis' put into 'priam' is 'PRIAPISM'.
'when' is the link.
(Another definition for priapism that I've seen is " male condition".)