Try a casual worker in borders of Tibet (7)
I believe the answer is:
attempt
'try' is the definition.
(I know that try can be written as attempt)
'a casual worker in borders of tibet' is the wordplay.
'casual worker' becomes 'temp' (short-term or temporary worker).
'in' means one lot of letters goes inside another.
'borders of' suggests removing the centre.
'tibet' with its centre taken out is 'tt'.
'temp' inserted inside 'tt' is 'ttempt'.
'a'+'ttempt'='ATTEMPT'
(Other definitions for attempt that I've seen before include "Try, endeavour" , "Have a crack" , "Effort; go" , "Undertake" , "Essay" .)