It is a cat mostly mistaken for a type of lion (7)
I believe the answer is:
asiatic
'a type of lion' is the definition.
'it is a cat mostly mistaken' is the wordplay.
'mostly' means to remove the last letter.
'mistaken' is an anagram indicator.
'it'+'is'+'a'+'cat'='itisacat'
'itisacat' with its last letter taken away is 'itisaca'.
'itisaca' anagrammed gives 'ASIATIC'.
'for' is the link.
(Other definitions for asiatic that I've seen before include "of Oriental origin" , "would include Chinese" , "Relating to one continent" , "Kind of beetle" , "Relating to the largest continent" .)