Almost entirely lecherous American writer (4)
I believe the answer is:
rand
'american writer' is the definition.
(Ayn Rand)
'almost entirely lecherous' is the wordplay.
'almost entirely' means to remove the last letter.
'lecherous' becomes 'randy' (both can mean sexually aroused).
'randy' with its last letter removed is 'RAND'.
(Other definitions for rand that I've seen before include "South African currency unit" , "Monetary unit in South Africa" , "South African money" , "our 1/4 Across [GOLD MEDALLIST] in 1964" , "Foreign currency" .)