Commando in protected area out of line calling in engineers (5,5)
I believe the answer is:
green beret
'commando' is the definition.
(I've seen this before)
'protected area out of line calling in engineers' is the wordplay.
'protected area' becomes 'green belt' (green area which cannot be built on).
'out of' indicates named letters should be taken away.
'line' becomes 'l' (used when specifying particular lines from a poem).
'calling in' is an insertion indicator.
'engineers' becomes 're' (Royal Engineers).
'greenbelt' with 'l' taken away is 'greenbet'.
'greenbet' going around 're' is 'GREEN BERET'.
'in' acts as a link.
(Other definitions for green beret that I've seen before include "British commando (colloq.)" , "soldier" , "Member of fighting unit" , "fighter in raiding unit" .)