Thin official entering shaft (6)
I believe the answer is:
rarefy
'thin' is the definition.
(I've seen this before)
'official entering shaft' is the wordplay.
'official' becomes 'ref' (match official in football, say).
'entering' indicates putting letters inside.
'shaft' becomes 'ray' (synonyms).
'ref' inserted inside 'ray' is 'RAREFY'.
(Other definitions for rarefy that I've seen before include "Make less solid; a ferry (anag.)" , "To make less dense" , "Make thinner" , "Fry are (anag.)" , "Become less dense" .)