Foreign character turns up in Paris park (3)
I believe the answer is:
psi
'foreign character' is the definition.
(Greek letter)
'turns up in paris park' is the wordplay.
'turns up' is a reversal indicator.
'in' indicates a hidden word.
'isp' can be found hidden inside 'paris park'.
'isp' reversed gives 'PSI'.
(Other definitions for psi that I've seen before include "Greek letter; pressure unit" , "Pounds per square inch (abbr.); Greek letter" , "Greek character" , "Pounds per sq. in. (abbr.)" , "letter from abroad" .)