The setter takes legal action about small problem (5)
I believe the answer is:
issue
'problem' is the definition.
(issue can mean a problem or difficulty)
'the setter takes legal action about small' is the wordplay.
'the setter' becomes 'I' (the person who set this crossword).
'takes' says to put letters next to each other.
'legal action' becomes 'sue' (as in suing in court).
'about' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sue' after 's' is 'ssue'.
'i' put next to 'ssue' is 'ISSUE'.
(Other definitions for issue that I've seen before include "Children, offspring" , "Supply" , "Continent" , "Matter of dispute" , "hot potato?" .)