Observed pole being cut (4)
I believe the answer is:
sawn
'being cut' is the definition.
(cut with a saw)
'observed pole' is the wordplay.
'observed' becomes 'saw' (seen).
'pole' becomes 'n' (abbreviation for North, as in North Pole).
'saw'+'n'='SAWN'
(Other definitions for sawn that I've seen before include "(Of wood) cut" , "went up and down?" , "Severed" , "Cut with serrated blade" , "timber must be" .)