Second broken toenail's bitten and sensitive (9)
I believe the answer is:
'sensitive' is the definition.
'second broken toenail's bitten' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'broken' is an anagram indicator.
'bitten' means one lot of letters goes inside another (inserted letters bite around others).
'toenail' with letters rearranged gives 'etional'.
'mo' going inside 'etional' is 'EMOTIONAL'.
'and' is the link.
(Other definitions for emotional that I've seen before include "Arousing strong feelings" , "Showing feelings like love or fear" , "Loving" , "Characterized by intense feeling" , "Very affected; upset" .)