Greek letter was first to be collected (5)
I believe the answer is:
piled
'be collected' is the definition.
(thesaurus)
'greek letter was first' is the wordplay.
'greek letter' becomes 'pi' (letter from the Greek alphabet).
'was first' becomes 'led'.
'pi'+'led'='PILED'
'to' acts as a link.
(Other definitions for piled that I've seen before include "put in a heap" , "Heaped" , "Arranged in stacks" .)