Complain of seeing rum blended (7)
I believe the answer is:
grumble
'complain' is the definition.
(I know that complain is a is a more specific form of the action of grumble)
'of seeing rum blended' is the wordplay.
'of' indicates a hidden word.
'GRUMBLE' can be found hidden inside 'seeing rum blended'.
(Other definitions for grumble that I've seen before include "Complaint" , "Complain, protest" , "Murmur with discontent" , "Grievance" , "Moan" .)