Strong urge (6)
(Other definitions for impulsion that I've seen before include "Force" , "Urge" .)
'strong urge' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yen that I've seen before include "change in East" , "Have a longing for Japanese money?" , "Longing (colloq.)" , "Intense longing" , "Japanese currency; yearning" .)
(Other definitions for desire that I've seen before include "Strong wish" , "Want strongly" , "Reside in longing" , "Crave" , "Want, wish for" .)