Rots the head off riders' seats (6)
I believe the answer is:
addles
'rots' is the definition.
The definition and answer can be both related to change as well as being verbs in their -s form.
Maybe they are linked in a way I don't understand?
'head off riders' seats' is the wordplay.
'head off' means to remove the first letter.
'riders' seats' becomes 'saddles' (I've seen this before).
'saddles' with its initial letter taken away is 'ADDLES'.
'the' is the link.
Can you help me to learn more?
(Other definitions for addles that I've seen before include "Confuses like bad egg" , "Makes rotten - makes muddled" , "Muddles, confuses" , "Confuses in saddle" , "Confuses or becomes rotten" .)