Pass round case of casual clothes (3)
I believe the answer is:
col
'pass' is the definition.
(col is a term for a mountain pass)
'round case of casual clothes' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'case of' says to hollow out the word (remove centre letters) (like emptying a case).
'clothes' means one lot of letters goes inside another (some letters clothe or cover others).
'casual' with its centre removed is 'cl'.
'o' inserted into 'cl' is 'COL'.
(Other definitions for col that I've seen before include "Young horse almost" , "A pass between mountain peaks" , "Lowest point of ridge between two peaks" , "Officer" , "Pass between hills" .)