Leading companion showing formality (6)
I believe the answer is:
starch
'formality' is the definition.
(I've seen this before)
'leading companion' is the wordplay.
'leading' becomes 'star' ('star' can be a synonym of 'leading').
'companion' becomes 'ch' (I've seen this in other clues).
'star'+'ch'='STARCH'
'showing' is the link.
(Other definitions for starch that I've seen before include "Stiffness of manner" , "Reserve" , "Stiffener in the laundry" , "Carbohydrate found in potatoes, wheat etc." , "Carbohydrate found in potatoes, wheat, and rice" .)