Elusive creature? One not seen thus far (3)
I believe the answer is:
yet
'thus far' is the definition.
(I've seen this before)
'elusive creature? one not' is the wordplay.
'elusive creature?' becomes 'yeti' (I've seen this in another clue).
'one' becomes 'I' (Roman numeral).
'not' indicates named letters should be taken away.
'yeti' with 'i' taken away is 'YET'.
'seen' is the link.
(Other definitions for yet that I've seen before include "Still; however" , "Still; nevertheless" , "Up till now" , "Although" , "in spite of that" .)