Celebrity companion you initially found to be rather formal (7)
I believe the answer is:
starchy
'formal' is the definition.
('starchy' can be similar in meaning to 'formal')
'celebrity companion you initially' is the wordplay.
'celebrity' becomes 'star' (I've seen this before).
'companion' becomes 'ch' (I've seen this in other clues).
'initially' says to take the initial letters.
The initial letter of 'you' is 'y'.
'star'+'ch'+'y'='STARCHY'
'found to be rather' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for starchy that I've seen before include "Unbending" , "Stiff, like potatoes" , "Stiff, formal" , "Conventional" , "Stiff and formalistic" .)