Firm breast not completely seen - breaking the watershed - it's stimulating (8)
I believe the answer is:
nicotine
'it's stimulating' is the definition.
(I know that Nicotine is stimulant)
'firm breast not completely seen breaking the watershed' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'breast' becomes 'tit' (informal term for a breast).
'not completely seen' means to remove the last letter.
'breaking' means one lot of letters goes inside another.
'the watershed' becomes 'nine' (nine o'clock is the UK TV watershed).
'co'+'tit'='cotit'
'cotit' with its final letter taken away is 'coti'.
'coti' going into 'nine' is 'NICOTINE'.
(Other definitions for nicotine that I've seen before include "Addictive drug" , "Not nice, I change to cigarette substance" , "Poison that occurs in tobacco" , "Tobacco element" , "I net coin from a popular poison" .)