Sign up fantastic panellist overlooking friend (6)
I believe the answer is:
enlist
'sign up' is the definition.
(enlisting is a kind of signing up)
'fantastic panellist overlooking friend' is the wordplay.
'fantastic' indicates an anagram (a fantastic or improbable ordering of the letters).
'overlooking' is a deletion indicator.
'friend' becomes 'pal' (pal can mean a friend).
'panellist' with 'pal' taken away is 'nelist'.
'nelist' anagrammed gives 'ENLIST'.
(Other definitions for enlist that I've seen before include "Enrol - recruit" , "Listen (anag.)" , "Join the armed services" , "augment army" , "Sign on -- 13 (anag)" .)