Telling Everyman: 'Wave Union flag, nothing to lose' (10)
I believe the answer is:
meaningful
'telling' is the definition.
(thesaurus)
'everyman wave union flag nothing to lose' is the wordplay.
'everyman' becomes 'me' (where 'Everyman' is the setter of the crossword).
'wave' indicates anagramming the letters (I've seen 'waving' mean this (letters wave around)).
'nothing' becomes 'O' (O resembles 0).
'to lose' indicates named letters should be taken away.
'union'+'flag'='unionflag'
'unionflag' with 'o' removed is 'uninflag'.
'uninflag' anagrammed gives 'aningful'.
'me'+'aningful'='MEANINGFUL'
(Other definitions for meaningful that I've seen before include "Important" , "Significant" .)