One propelled with the speed of a shell? (4)
I believe the answer is:
boat
'speed of a shell?' is the definition.
'boat' can be an answer for 'shell?' (shell is a kind of boat). I am unsure of the 'speed of a' bit.
'one propelled with the' is the wordplay.
'one propelled with' becomes 'boa' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'boa'+'t'='BOAT'
Can you help me to learn more?
(Other definitions for boat that I've seen before include "Contest on the Thames" , "Vessel for travelling on water" , "Small ship" , "See 12 Down" , "See 18" .)