Join lines scribbled on tabletop (6)
I believe the answer is:
enlist
'join' is the definition.
(I know that enlist is a kind of sign up)
'lines scribbled on tabletop' is the wordplay.
'scribbled' is an anagram indicator (ie written badly).
'on' says to put letters next to each other.
'tabletop' becomes 't' (top of the word 'table').
'lines' with letters rearranged gives 'enlis'.
'enlis'+'t'='ENLIST'
(Other definitions for enlist that I've seen before include "Join the military" , "Enrol in armed forces" , "Sign on -- 13 (anag)" , "Obtain help of" , "Enrol in the services" .)