Number one component
I believe the answer is:
unit
This clue is a triple definition!
'number' is the first definition.
(unit is a kind of number)
'one' is the second definition.
(synonyms)
'component' is the third definition.
(I've seen this in another clue)
(Other definitions for unit that I've seen before include "Single undivided entity" , "Mexican maize pancake" , "Single thing or person" , "Some fighting men" , "Part of a kit" .)