Turning up at one clutching love letter from Greece (4)
I believe the answer is:
iota
'letter from greece' is the definition.
(letter from the Greek alphabet)
'turning up at one clutching love' is the wordplay.
'turning up' shows that the letters should be reversed in order.
'one' becomes 'i' (Roman numeral).
'clutching' indicates putting letters inside.
'love' becomes 'o' (love means zero in tennis).
'at'+'i'='ati'
'ati' enclosing 'o' is 'atoi'.
'atoi' back-to-front is 'IOTA'.
(Other definitions for iota that I've seen before include "Old letter" , "Jot or tittle" , "Trace" , "Greek equivalent of the letter I" , "little part" .)