At home this person’s invested in fast pain relief (8)
I believe the answer is:
liniment
'pain relief' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'at home this person's invested in fast' is the wordplay.
'at home' becomes 'in' (similar in meaning).
'this person's' becomes 'im' (this person means the crossword setter themselves).
'invested in' is an insertion indicator.
'fast' becomes 'lent' (I've seen this before).
'in'+'im'='inim'
'inim' put into 'lent' is 'LINIMENT'.
Can you help me to learn more?
(Other definitions for liniment that I've seen before include "Lotion" , "Liquid for rubbing into skin" , "Embrocation" , "application" , "Medicinal liquid for the muscles" .)