Caution mostly exercised, holding dull-looking gun (9)
I believe the answer is:
automatic
'looking gun' is the definition.
'automatic' can be an answer for 'gun' (automatic is a kind of gun). I am unsure of the 'looking' bit.
'caution mostly exercised holding dull' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'exercised' indicates anagramming the letters.
'holding' means one lot of letters goes inside another.
'dull' becomes 'mat' (mat as opposed to glossy).
'caution' with its final letter taken off is 'cautio'.
'cautio' anagrammed gives 'autoic'.
'autoic' going around 'mat' is 'AUTOMATIC'.
Can you help me to learn more?
(Other definitions for automatic that I've seen before include "Without volition or conscious control" , "Type of gun or car" , "Like George" , "Occurring as a matter of course" , "like a robot" .)