clearance vs headroom

clearance and headroom both are nouns.

Word	Noun	Adjective	Verb	Adverb
clearance	Yes	No	No	No
headroom	Yes	No	No	No

As nouns, clearance and headroom are synonyms defined as:

clearance and headroom: vertical space available to allow easy passage under something

Other synonyms of clearance include headway.

clearance (noun)	headroom (noun)
permission to proceed	the capacity of a system to reproduce loud sounds without distortion
the distance by which one thing clears another; the space between them	vertical space available to allow easy passage under something
vertical space available to allow easy passage under something