An animal in bed (one having taken Viagra finally) (5)
I believe the answer is:
coati
'an animal' is the definition.
(mammal of Central and South America)
'bed one having taken viagra finally' is the wordplay.
'bed' becomes 'cot' (cot is a kind of bed).
'one' becomes 'I' (Roman numeral).
'having taken' means one lot of letters goes inside another.
'finally' indicates one should take the final letters.
The last letter of 'viagra' is 'a'.
'cot'+'i'='coti'
'coti' going around 'a' is 'COATI'.
'in' is the link.
(Other definitions for coati that I've seen before include "beast" , "carnivore from South America" , "Flesh-eating mammal" , "Raccoon relative" , "omnivorous mammal" .)