Strong desire or impulse (4)
I believe the answer is:
urge
I believe this clue is a double definition.
'strong desire' is the first definition.
(I've seen this before)
'impulse' is the second definition.
(synonyms)
(Other definitions for urge that I've seen before include "Incite to action" , "Advise earnestly, prod" , "Press; drive" , "Encourage (to do something)" , "Itch" .)