The pest is given a leg up, mistakenly (6)
I believe the answer is:
plague
'the pest' is the definition.
(thesaurus)
'given a leg up mistakenly' is the wordplay.
'given' says to put letters next to each other.
'mistakenly' indicates an anagram.
'a'+'leg'='aleg'
'aleg' put after 'up' is 'upaleg'.
'upaleg' with letters rearranged gives 'PLAGUE'.
'is' is the link.
(Other definitions for plague that I've seen before include "Epidemic diseases with high mortality" , "large scale affliction" , "Swarm of insects" , "Pestilence transmitted by rat fleas" , "Deadly contagious disease, or a swarm of locusts" .)