Men with horns closest to fight mythical creature (4,4,7)
I believe the answer is:
loch ness monster
'mythical creature' is the definition.
(the Loch Ness Monster is a mythical monster)
'men with horns closest to fight' is the wordplay.
'with' says to put letters next to each other.
'to fight' indicates an anagram (I've seen 'fighting' mean this (I've seen this in other clues)).
'men'+'horns'+'closest'='menhornsclosest'
'menhornsclosest' anagrammed gives 'LOCH NESS MONSTER'.
(Other definitions for loch ness monster that I've seen before include "lt's never been found" , "Caledonian enigma" , "in the Caledonian Canal" , "chimera?" , "Scots apparition" .)