Tickle bird, badly worried (9)
I believe the answer is:
titillate
'tickle' is the definition.
('titillate' can be a synonym of 'tickle')
'bird badly worried' is the wordplay.
'bird' becomes 'tit' (I've seen this before).
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'worried' becomes 'ate' (something worrying may eat someone up).
'tit'+'ill'+'ate'='TITILLATE'
(Other definitions for titillate that I've seen before include "arouse interest" , "Stimulate agreeably" , "Gently stimulate" , "Excite erotically" , "Tickle" .)