Reptile unsettled a girl a lot (9)
I believe the answer is:
alligator
'reptile' is the definition.
(alligator is a kind of reptile)
'unsettled a girl a lot' is the wordplay.
'unsettled' indicates an anagram.
'a'+'girl'+'a'+'lot'='agirlalot'
'agirlalot' with letters rearranged gives 'ALLIGATOR'.
(Other definitions for alligator that I've seen before include "Reptile like a crocodile" , "What can go with pear" , "River creature" , "It could be in zoo" , "Predatory cold-blooded killer" .)