Clasp securing tot's protective covering (8)
I believe the answer is:
cladding
'protective covering' is the definition.
(cladding is a kind of protective covering)
'clasp securing tot's' is the wordplay.
'clasp' becomes 'cling' (I am not sure about this - if you are sure you should believe this answer much more).
'securing' means one lot of letters goes inside another.
'tot' becomes 'add' (synonyms).
'cling' placed around 'add' is 'CLADDING'.
Can you help me to learn more?
(Other definitions for cladding that I've seen before include "protective outer covering" , "Protective covering on a building" , "Covering material" , "Outside wall covering" , "Exterior covering" .)