Poisonous, though it adds a bit of spice to a kiss! (5)
I believe the answer is:
toxic
'poisonous' is the definition.
(synonyms)
'it adds a bit of spice to a kiss' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'bit' could be 'o' (resembles 0, a binary digit or bit) and 'o' is found within the answer.
'kiss' could be 'x' (X can represent a kiss) and 'x' is found within the answer.
This explanation may well be incorrect...
'though' acts as a link.
Can you help me to learn more?
(Other definitions for toxic that I've seen before include "Deadly - venomous" , "Harmful - cox it (anag)" , "Poisonous, noxious" , "word describing bad things" .)