Really hard work to get article's strapline (6)
'strapline' is the definition.
'really hard work to get article's' is the wordplay.
'really hard work' becomes 'slog' (slog can informally mean hard or boring work).
'to get' means one lot of letters go next to another.
'article' becomes 'an' (indefinite article).
(Other definitions for slogan that I've seen before include "Memorable advertising phrase", "Marketing phrase", "'Motto, watchword (6)'", "Logo", "War cry - advertising jingle".)