Caustic and resentful on retirement, hard to leave scene of activity (7)
I believe the answer is:
erosive
'caustic' is the definition.
(synonyms)
'resentful on retirement hard to leave scene of activity' is the wordplay.
'resentful' becomes 'sore' (sore can mean resentful or annoyed).
'on retirement' says the letters should be written in reverse.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to leave' indicates named letters should be taken away.
'scene of activity' becomes 'hive' (I've seen this before).
'sore' backwards is 'eros'.
'hive' with 'h' taken out is 'ive'.
'eros'+'ive'='EROSIVE'
'and' is the link.
(Other definitions for erosive that I've seen before include "Wearing away" , "Tending to wear away" .)