Best old radio receiver part installed in most of these (3,4,8)
I believe the answer is:
the cats whiskers
'best' is the definition.
(the cat's whiskers can informally mean the best)
'old radio receiver part installed in most of these' is the wordplay.
'old radio receiver part' becomes 'cat's whisker' (piece of fine wire in an early radio).
'installed in' is an insertion indicator.
'most of' means to remove the last letter.
'these' with its final letter removed is 'thes'.
'catswhisker' inserted within 'thes' is 'THE CATS WHISKERS'.
(Other definitions for the cats whiskers that I've seen before include "Superb - vibrissae" , "Exactly what's wanted" , "item of early radio equipment?" , "The best thing" , "Rectifiers" .)