Sack is small, having dull exterior (9)
I believe the answer is:
dismissal
'sack' is the definition.
(sacking is dismissal from one's job)
'is small having dull exterior' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'having' means one lot of letters go next to another.
'dull' becomes 'dismal' (I am not sure about this - if you are sure you should believe this answer much more).
'exterior' indicates putting letters inside (some letters go on the outside of others).
'is'+'s'='iss'
'iss' placed into 'dismal' is 'DISMISSAL'.
Can you help me to learn more?
(Other definitions for dismissal that I've seen before include "Termination of employment" , "sack" , "brush-off" , "Sacking; end of innings" , "Rejection" .)