Mouser links up with rat, filling new roles (7)
I believe the answer is:
casting
'mouser links up' is the definition.
I can't tell whether this defines the answer.
'with rat filling new roles' is the wordplay.
'with' means one lot of letters go next to another.
'rat' becomes 'scab' (synonyms).
'filling' means to look at the middle letters (the letters which fill the word).
'new roles' becomes 'sting' (I am not sure about this - if you are sure you should believe this answer much more).
The middle letters of 'scab' are 'ca'.
'ca' put next to 'sting' is 'CASTING'.
Can you help me to learn more?
(Other definitions for casting that I've seen before include "Choosing actors" , "Shaping metal in mould" , "shell across back of fort" , "The process of selecting actors for play" , "Shaping in mould; assigning actors to" .)