A pert sort at work, lying (9)
I believe the answer is:
prostrate
'lying' is the definition.
(thesaurus)
'a pert sort at work' is the wordplay.
'at work' indicates an anagram.
'a'+'pert'+'sort'='apertsort'
'apertsort' with letters rearranged gives 'PROSTRATE'.
(Other definitions for prostrate that I've seen before include "Prone" , "Lying stretched out" , "Lying face downwards" , "Lying face-down" , "Flat" .)