Effective restraint in German plane (6)
I believe the answer is:
glider
'plane' is the definition.
(I've seen this before)
'effective restraint in german' is the wordplay.
'effective restraint' becomes 'lid' (as in putting a lid on something).
'in' indicates putting letters inside.
'german' becomes 'ger' (abbreviation).
'lid' going into 'ger' is 'GLIDER'.
(Other definitions for glider that I've seen before include "Girdle for silent flier" , "Plane supported by air" , "taking flight?" , "quiet flier" , "Aircraft supported solely by air" .)