Sly bird, tense, having timid exterior (8)
I believe the answer is:
stealthy
'sly' is the definition.
(thesaurus)
'bird tense having timid exterior' is the wordplay.
'bird' becomes 'teal' (teal is a kind of bird).
'tense' becomes 't' (abbreviation used in many dictionaries).
'having' is a charade indicator (letters next to each other).
'timid' becomes 'shy' (synonyms).
'exterior' is an insertion indicator.
'teal'+'t'='tealt'
'tealt' going within 'shy' is 'STEALTHY'.
(Other definitions for stealthy that I've seen before include "Moving surreptitiously" , "'Furtive, surreptitious (8)'" , "Secretive, silent" , "Acting in furtive or covert manner" , "Covert - clandestine" .)