Wanting a sea shell — blowing top to get tune (8)
I believe the answer is:
serenade
'top to get tune' is the definition.
I can't judge whether this definition defines the answer.
'wanting a sea shell blowing' is the wordplay.
'wanting a sea' becomes 'serena' (I am not sure about this - if you are sure you should believe this answer much more).
'shell' means to remove the middle letters (I've seen 'shell of' mean this).
'blowing' becomes 'drifting with the tide' (drifting with the tide is a kind of blowing).
'driftingwiththetide' with its centre taken out is 'de'.
'serena'+'de'='SERENADE'
Can you help me to learn more?
(Other definitions for serenade that I've seen before include "It's played or sung at a lover's window" , "Music appropriate for evening" , "Murder" , "Song of romance" , "Open-air song" .)