A sleeping place for the big rats outside! (5)
I believe the answer is:
roost
'a sleeping place' is the definition.
(thesaurus)
'the big rats outside' is the wordplay.
'the big' becomes 'os'.
'rats' becomes 'rot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'outside' means one lot of letters goes inside another.
'os' placed inside 'rot' is 'ROOST'.
'for' is the link.
Can you help me to learn more?
(Other definitions for roost that I've seen before include "Hen house; perch" , "somewhere to sleep" , "Settle down (on a branch?)" , "Hen's resting place" , "Rest on a perch" .)