Turns slap into mountains (4)
I believe the answer is:
alps
'mountains' is the definition.
(European mountain range)
'turns slap' is the wordplay.
'turns' is an anagram indicator.
'slap' anagrammed gives 'ALPS'.
'into' acts as a link.
(Other definitions for alps that I've seen before include "Mountains in Central Europe" , "Slopes" , "European mountains" , "Mountain system in south-central Europe" , "high points" .)