Observant, but terribly tentative (9)
I believe the answer is:
attentive
'observant' is the definition.
(similar in meaning)
'terribly tentative' is the wordplay.
'terribly' indicates an anagram.
'tentative' anagrammed gives 'ATTENTIVE'.
'but' acts as a link.
(Other definitions for attentive that I've seen before include "Careful" , "Heedful, thoughtful" , "Concentrating; politely helpful" , "Alert; observant" , "Taking notice" .)