Small component and mostly simple (7)
I believe the answer is:
spartan
'simple' is the definition.
(I've seen this before)
'small component and mostly' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'component' becomes 'part' (a component is a part of something).
'mostly' means to remove the last letter.
'and' with its final letter taken off is 'an'.
's'+'part'+'an'='SPARTAN'
(Other definitions for spartan that I've seen before include "Bleak - rigorous - austere" , "Severe" , "Rather austere" , "Not comfortable" , "Indifferent to comfort" .)