Figure student must possess a popular antiseptic (8)
I believe the answer is:
formalin
'antiseptic' is the definition.
(I've seen this before)
'figure student must possess a popular' is the wordplay.
'figure' becomes 'form' (synonyms).
'student' becomes 'L' (as in L-plates for learner drivers).
'must possess' indicates putting letters inside.
'popular' becomes 'in' ('in' can mean trendy).
'a'+'in'='ain'
'l' inserted within 'ain' is 'alin'.
'form'+'alin'='FORMALIN'
(Other definitions for formalin that I've seen before include "antiseptic" , "Disinfectant" , "Preservative" , "preserving solution" , "embalming fluid" .)