Unable to see colleague crossing far side of wide cul-de-sac (5,5)
I believe the answer is:
blind alley
'cul-de-sac' is the definition.
('blind alley' can be a synonym of 'cul-de-sac')
'unable to see colleague crossing far side of wide' is the wordplay.
'unable to see' becomes 'blind' (I've seen this before).
'colleague' becomes 'ally' (I've seen this before).
'crossing' indicates putting letters inside.
'far side of' suggests the final letters.
The final letter of 'wide' is 'e'.
'ally' enclosing 'e' is 'alley'.
'blind'+'alley'='BLIND ALLEY'
(Other definitions for blind alley that I've seen before include "Cul-de-sac" , "no escape route" , "no progress there" , "dead-end position" .)