Second customer calling for house cat (6)
I believe the answer is:
Here is my best explanation:
'house cat' is the definition.
(mouser is a kind of house cat)
'second customer calling' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'customer calling' becomes 'user' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for' acts as a link.