Lip in great pain, working in a filthy plant (8)

I believe the answer is:

agrimony

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'working in a filthy plant' is the definition.
'agrimony' can be an answer for 'plant' (agrimony is a kind of plant). I'm not sure about the 'working in a filthy' bit.

'lip in great pain' is the wordplay.
'lip' becomes 'rim' (synonyms).
'in' indicates putting letters inside.
'great pain' becomes 'agony' (I've seen this before).
'rim' put into 'agony' is 'AGRIMONY'.

Can you help me to learn more?

(Other definitions for agrimony that I've seen before include "Plant with yellow flower-spikes" , "Growth" .)

I've seen this clue in The Times.

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