Real nests lost attention (9)
I believe the answer is:
alertness
'attention' is the definition.
(I know that alertness is a type of attention)
'real nests lost' is the wordplay.
'lost' indicates an anagram.
'real'+'nests'='realnests'
'realnests' is an anagram of 'ALERTNESS'.
(Other definitions for alertness that I've seen before include "It's needed to respond quickly to danger" , "Watchful attitude" , "Showing attentive nature" , "Wakeful characteristic" , "Watchful state" .)