Sibilant sound in Spanish is soft (4)
I believe the answer is:
hiss
'sibilant sound' is the definition.
(I've seen this before)
'in spanish is soft' is the wordplay.
'in' says the answer is hidden in the clue.
'HISS' is hidden within 'spanish is soft'.
(Other definitions for hiss that I've seen before include "Sibilant sound made through the teeth" , "S-sound of disapproval" , "Catcall" , "less welcome sign?" , "Snake's sibilant sound" .)