Beam: at first surveyor measures its length exactly (5)
I believe the answer is:
smile
'beam' is the definition.
(thesaurus)
'first surveyor measures its length exactly' is the wordplay.
'first' indicates taking the first letters.
The initial letters of 'surveyor measures its length exactly' is 'SMILE'.
'at' acts as a link.
(Other definitions for smile that I've seen before include "express pleasure" , "Pleased expression" , "Visually express amusement" , "Show you're pleased" , "Facial expression showing pleasure" .)