... briefly contemplate turning lecherous ... (4)
I believe the answer is:
lewd
'lecherous' is the definition.
(thesaurus)
'briefly contemplate turning' is the wordplay.
'briefly' means to remove the last letter.
'contemplate' becomes 'dwell' (dwelling is a kind of contemplating).
'turning' shows that the letters should be reversed in order.
'dwell' with its last letter taken off is 'dwel'.
'dwel' written backwards gives 'LEWD'.
(Other definitions for lewd that I've seen before include "Crude, offensive" , "Obscene - lustful - sensual" , "Weld in lascivious way" , "Salacious" , "Indecent, licentious" .)