Horrid ants wandering closer to formicary (5)
I believe the answer is:
nasty
'horrid' is the definition.
(I've seen this before)
'ants wandering closer to formicary' is the wordplay.
'wandering' is an anagram indicator.
'closer to' suggests the final letters (the letter which ends or closes the word).
The final letter of 'formicary' is 'y'.
'ants' anagrammed gives 'nast'.
'nast'+'y'='NASTY'
(Other definitions for nasty that I've seen before include "Obscene" , "Spiteful and unkind" , "grave" , "Vile" , "Horrible" .)