Fell back after principal gets clearance (8)
I believe the answer is:
headroom
'clearance' is the definition.
('headroom' can be a synonym of 'clearance')
'fell back after principal' is the wordplay.
'fell' becomes 'moor' (I've seen this before).
'back' shows that the letters should be reversed in order.
'after' means one lot of letters go next to another.
'principal' becomes 'head' ('head' can be a synonym of 'principal'**).
'moor' written backwards gives 'room'.
'room' after 'head' is 'HEADROOM'.
'gets' is the link.
(Other definitions for headroom that I've seen before include "Space available above" , "vertical space" , "enough space" , "Clearance below a bridge or ceiling" , "Clearance vertically" .)