Saw the fellow left between the sheets? (6)
I believe the answer is:
beheld
'saw' is the definition.
(beholding is a kind of seeing)
'the fellow left between the sheets?' is the wordplay.
'the fellow' becomes 'he' (I've seen this in other clues).
'left' becomes 'l' (common abbreviation).
'between' is an insertion indicator.
'the sheets?' becomes 'bed'.
'he'+'l'='hel'
'hel' inserted within 'bed' is 'BEHELD'.
(Other definitions for beheld that I've seen before include "Laid eyes on" , "Seen" , "Saw or observed in olden times" , "watched" .)