Functioned as (3)
I believe the answer is:
was
'functioned as' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for was that I've seen before include "'Is, in the past (3)'" , "Lived" , "(I/he) existed" , "(I/he/she/it) existed" , "(He/she/it) existed" .)