Anxious, perhaps even worried about one (12)
I believe the answer is:
apprehensive
'anxious' is the definition.
(I've seen this before)
'perhaps even worried about one' is the wordplay.
'worried' indicates an anagram.
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'perhaps'+'even'='perhapseven'
'perhapseven' is an anagram of 'apprehensve'.
'apprehensve' enclosing 'i' is 'APPREHENSIVE'.
(Other definitions for apprehensive that I've seen before include "Feeling anxiety or fear about future events" , "Uneasy, anxious" , "Fearful, anxious" , "Worried" , "Seven happier (anag.)" .)