Try a casual worker in borders of Tibet (7)
I believe the answer is:
attempt
'try' is the definition.
(synonyms)
'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' means to remove the middle letters.
'tibet' with its middle taken out is 'tt'.
'temp' going into '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" .)