Tendency to break nipper's toy (10)
I believe the answer is:
propensity
'tendency' is the definition.
(I know that propensity is a type of tendency)
'break nipper's toy' is the wordplay.
'break' indicates an anagram.
'nippers'+'toy'='nipperstoy'
'nipperstoy' anagrammed gives 'PROPENSITY'.
'to' is the link.
(Other definitions for propensity that I've seen before include "Inclination; tendency" , "Bent" .)