Plot uneven with novel's ending held to be ''messy'' (10)
I believe the answer is:
bedraggled
'be 'messy'' is the definition.
(thesaurus)
'plot uneven with novel's ending held' is the wordplay.
'plot' becomes 'bed' (a plot of land).
'uneven' becomes 'ragged' (similar in meaning).
'with' says to put letters next to each other.
'ending' suggests the final letters (the ending of the word).
'held' means one lot of letters goes inside another.
The last letter of 'novel' is 'l'.
'bed'+'ragged'='bedragged'
'bedragged' going around 'l' is 'BEDRAGGLED'.
'to' acts as a link.
(Other definitions for bedraggled that I've seen before include "muddy and dripping" , "Unkempt" , "Very untidy" , "Dishevelled" , "Wet and dirty" .)