Saw, only to become a prisoner (6)
I believe the answer is:
beheld
'saw' is the definition.
(beholding is a kind of seeing)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for beheld that I've seen before include "watched" , "Seen" , "Laid eyes on" , "Saw or observed in olden times" .)