Complaint harboured by Anthea on meeting Rachel (7)
I believe the answer is:
earache
'anthea on meeting rachel' is the definition.
The definition suggests a singular noun which matches the answer.
'complaint harboured by' is the wordplay.
'complaint' becomes 'ache'.
'harboured' becomes 'ear' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'by' says to put letters next to each other.
'ache' put after 'ear' is 'EARACHE'.
Can you help me to learn more?
(Other definitions for earache that I've seen before include "complaint to The Listener" , "Otic pain" , "Otalgia" , "Aural discomfort" , "Pain for listener" .)