Stick a short gun in more efficiently when sawn-off (8)
I believe the answer is:
baguette
'stick' is the definition.
(a stick of bread)
'a short gun in more efficiently when sawn-off' is the wordplay.
'short' means to remove the last letter.
'in' means one lot of letters goes inside another.
'more efficiently' becomes 'better'.
'when sawn-off' means to remove the last letter (the final letter is cut off).
'gun' with its last letter taken away is 'gu'.
'a'+'gu'='agu'
'better' with its last letter taken away is 'bette'.
'agu' inserted into 'bette' is 'BAGUETTE'.
(Other definitions for baguette that I've seen before include "Long French breadstick" , "A long thin French loaf" , "Long narrow French loaf" , "Bread roll" , "Narrow French stick loaf" .)