Second customer calling for house cat (6)
I believe the answer is:
mouser
'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).
'mo'+'user'='MOUSER'
'for' is the link.
Can you help me to learn more?
(Other definitions for mouser that I've seen before include "Predatory cat" , "Cat as hunter of small rodents" , "pest controller" , "Rodent catcher" , "Cat with certain skills" .)