*Instrument mobile on gaining wheel? (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'mobile on gaining wheel?' is the wordplay.
'mobile' becomes 'cell' (a cell phone).
'on' says to put letters next to each other.
'gaining wheel?' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'cell'+'o'='CELLO'
Can you help me to learn more?
(Other definitions for cello that I've seen before include "has strings attached" , "Musical instrument like large violin held between legs" , "large-scale fiddle" , "Played by Rostropovich" , "Tortelier's instrument" .)