A friend covered with very large gems (5)
I believe the answer is:
opals
'gems' is the definition.
(opal is a kind of gem)
'a friend covered with very large' is the wordplay.
'a friend' becomes 'pal' (pal can mean a friend).
'covered with' is an insertion indicator.
'very large' becomes 'os' (abbreviation for oversize).
'pal' put into 'os' is 'OPALS'.
(Other definitions for opals that I've seen before include "Milky white and coloured gems" , "jewellery" , "Milky sparkling gemstones" , "jewels" , "Precious stones" .)