Left in pain by my quasi-science (7)
I believe the answer is:
alchemy
'science' is the definition.
(I have seen 'old science' mean 'alchemy' so perhaps 'science' could also mean 'alchemy')
'left in pain by my' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'pain' becomes 'ache' (ache is a kind of pain).
'by' is a charade indicator (letters next to each other).
'l' going inside 'ache' is 'alche'.
'alche'+'my'='ALCHEMY'
'quasi' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for alchemy that I've seen before include "Philosopher's Stone its object" , "Medieval forerunner of chemistry" , "Search for gold and philosopher's stone" , "Pre-scientific chemistry" , "Chemistry's forerunner" .)