Forceful companion, weak-looking on the outside (6)
I believe the answer is:
punchy
'forceful' is the definition.
(I've seen this before)
'companion weak-looking on the outside' is the wordplay.
'companion' becomes 'ch' (I've seen this in other clues).
'weak-looking' becomes 'puny'.
'on the outside' means one lot of letters goes inside another.
'ch' placed inside 'puny' is 'PUNCHY'.
(Other definitions for punchy that I've seen before include "Hard-hitting" , "Sharp and powerful" , "Forceful" , "Cutting" , "Incisive" .)