More erotic guide describing second threesome of bigamists (8)
I believe the answer is:
steamier
'more erotic' is the definition.
(I've seen this in another clue)
'guide describing second threesome of bigamists' is the wordplay.
'guide' becomes 'steer' (synonyms).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'second threesome of bigamists' becomes 'ami' (second set of three letters from 'bigamists').
'steer' placed around 'ami' is 'STEAMIER'.
(Other definitions for steamier that I've seen before include "Comparatively hot and humid" , "more like a sauna" .)