Second customer calling for house cat (6)
I believe the answer is:
'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 can't explain this - if you can you should give a lot more credence to this answer).
'for' is the link.
Can you help me to learn more?
(Other definitions for mouser that I've seen before include "moggy", "Cat as hunter of small rodents", "One getting rid of vermin", "Cat proficient at catching small rodents", "Preying cat".)