Complain as you send back a double tot of spirits (6)
I believe the answer is:
murmur
'complain' is the definition.
(I know this)
'back a double tot of spirits' is the wordplay.
'back' shows that the letters should be reversed in order.
'a double tot of' becomes 'rum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'spirits' becomes 'rum' (rum is a kind of spirits).
'rum'+'rum'='rumrum'
'rumrum' in reverse letter order is 'MURMUR'.
'as you send' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for murmur that I've seen before include "Low continuous sound" , "Mumble" , "Little noise" , "Indistinct mutter" , "Low background noise" .)