Not the first character to have instinct (6,6)
I believe the answer is:
second nature
'have instinct' is the definition.
The definition suggests a singular noun which matches the answer.
'not the first character' is the wordplay.
'not the first' becomes 'second' (not the first but the second).
'character' becomes 'nature' (I've seen this before).
'second'+'nature'='SECOND NATURE'
'to' is the link.
Can you help me to learn more?
(Other definitions for second nature that I've seen before include "it's become instinctive" , "Almost instinctive behaviour" , "Deeply ingrained tendency" , "Apparently innate behaviour" , "acquired habit?" .)