Very old, hot-headed complaints (5)
I believe the answer is:
hills
'very old' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'hot-headed complaints' is the wordplay.
'hot-headed' becomes 'h' (this might be a standard abbreviation I've not previously seen).
'complaints' becomes 'ills' ('ill' can be a synonym of 'complaint').
'h'+'ills'='HILLS'
Can you help me to learn more?
(Other definitions for hills that I've seen before include "Elevations of land" , "Mounts" , "Heights; tors" , "High terrain" , "See 7" .)