Male animal in discomfort beginning to emit unpleasant chemical (8)
I believe the answer is:
ptomaine
'unpleasant chemical' is the definition.
(unpleasant tasting chemical compound)
'male animal in discomfort beginning to emit' is the wordplay.
'male animal' becomes 'tom' (male cat).
'in' means one lot of letters goes inside another.
'discomfort' becomes 'pain' (I have seen 'Physical discomfort ' mean 'pain' so perhaps 'discomfort' could also mean 'pain').
'beginning to' suggests taking the first letters.
The first letter of 'emit' is 'e'.
'tom' going within 'pain' is 'ptomain'.
'ptomain'+'e'='PTOMAINE'
(Other definitions for ptomaine that I've seen before include "Sort of poisoning" , "A pimento might turn to food poisoning" , "Consequence of decay" , "Pain to me (anag.)" , "poison!" .)