Sing while greeting Derek (5)
I believe the answer is:
yodel
'sing while' is the definition.
'yodel' can be an answer for 'sing' (yodeling is a kind of singing). I am unsure of the 'while' bit.
'greeting derek' is the wordplay.
'greeting' becomes 'yo'.
'derek' becomes 'del' (I can't explain this - if you can you should give a lot more credence to this answer).
'yo'+'del'='YODEL'
Can you help me to learn more?
(Other definitions for yodel that I've seen before include "Sing in Tyrolean fashion" , "Sing (Alpine folk music)" , "Loftily call" , "make an off-peak call" , "Sing warblingly" .)