Being very old, they make complaints at Southend (5)
I believe the answer is:
hills
'being very old they' is the definition.
Although both the answer and 'being' are verbs, they are not in the same form of the verb.
'complaints at southend' is the wordplay.
'complaints' becomes 'ills' ('ill' can be a synonym of 'complaint').
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'southend' becomes 'h' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ills' after 'h' is 'HILLS'.
'make' is the link.
Can you help me to learn more?
(Other definitions for hills that I've seen before include "Mounts" , "Elevations of land" , "High terrain" , "Heights; tors" , "See 7" .)