headroom vs clearance

headroom and clearance both are nouns.

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

As nouns, headroom and clearance are synonyms defined as:

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

Other synonyms of headroom include headway.

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