Folklore monster making Tom's head go round (5)
I believe the answer is:
'folklore monster' is the definition.
(I've seen this before)
'tom's head go round' is the wordplay.
'head' suggests taking the first letters.
'go round' becomes 'roll' (I've seen this in other clues).
The first letter of 'toms' is 't'.
'making' acts as a link.
(Other definitions for troll that I've seen before include "Cyber pest" , "cyber-bully" , "Malignant elf" , "Dwarf (or giant) of Norse myth" , "Evil dwarf" .)