Time short with favourite instrument (7)
I believe the answer is:
trumpet
'instrument' is the definition.
(trumpet is a kind of instrument)
'time short with favourite' is the wordplay.
'time' becomes 't'.
'short' becomes 'rum' (short can mean an alcoholic drink**).
'with' means one lot of letters go next to another.
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
't'+'rum'+'pet'='TRUMPET'
(Other definitions for trumpet that I've seen before include "Horn" , "beastly noise" , "Shout!" , "Centre of daffodil flower" , "brassy type" .)