Employ skilled person to remove phosphorus (5)
I believe the answer is:
exert
'employ' is the definition.
(eg exerting strength is using or employing it)
'skilled person to remove phosphorus' is the wordplay.
'skilled person' becomes 'expert' (someone with expertise or skills).
'to remove' is a deletion indicator.
'phosphorus' becomes 'P' (P is the chemical symbol for phosphorus).
'expert' with 'p' taken out is 'EXERT'.
(Other definitions for exert that I've seen before include "Strain" , "Bring into use, apply" , "Employ" , "Act upon" , "Apply (force)" .)