Stick with a round exterior (3)
I believe the answer is:
rod
'stick' is the definition.
(I've seen this before)
'with a round exterior' is the wordplay.
'with' is an insertion indicator.
'a round' becomes 'o' (round can mean a round object).
'exterior' becomes 'rd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'o' put into 'rd' is 'ROD'.
Can you help me to learn more?
(Other definitions for rod that I've seen before include "Fisherman's item" , "Angler's pole" , "Flexible stick for 11 Down" , "Staff" , "Measurement" .)