Goes for son in bed with temptress at last (5)
I believe the answer is:
costs
'goes for' is the definition.
(I've seen this in another clue)
'son in bed with temptress at last' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in' means one lot of letters goes inside another.
'bed' becomes 'cot' (cot is a type of bed).
'with' is a charade indicator (letters next to each other).
'at last' suggests the final letters.
The last letter of 'temptress' is 's'.
's' inserted within 'cot' is 'cost'.
'cost'+'s'='COSTS'
(Other definitions for costs that I've seen before include "Is priced at" , "Expenditure" , "Charges" , "Overheads" , "prices" .)