Finally he let the guys inside for a component (7)
I believe the answer is:
element
'for a component' is the definition.
('element' can be a synonym of 'component')
'finally he let the guys inside' is the wordplay.
'finally' indicates one should take the final letters.
'the guys' becomes 'men' (guy can mean a man).
'inside' is an insertion indicator.
The last letter of 'he' is 'e'.
'let' enclosing 'men' is 'lement'.
'e'+'lement'='ELEMENT'
(Other definitions for element that I've seen before include "Ingredient - small amount" , "Iron, for instance - part of an electric kettle" , "Constituent -- habitat" , "Fundamental part of whole" , "Trace" .)