Head of CGT in foreign union is a court official (6)
I believe the answer is:
nuncio
'court official' is the definition.
'nuncio' can be an answer for 'official' (nuncio is a kind of official). I'm not sure about the 'court' bit.
'head of cgt in foreign union' is the wordplay.
'head of' says to take the initial letters.
'in' means one lot of letters goes inside another.
'foreign' is an anagram indicator (foreign order of the letters).
The initial letter of 'cgt' is 'c'.
'union' is an anagram of 'nunio'.
'c' inserted within 'nunio' is 'NUNCIO'.
'is a' is the link.
Can you help me to learn more?
(Other definitions for nuncio that I've seen before include "See man" , "Sent by the Pope" , "Papal ambassador" , "Vatican delegate" , "Papal emissary" .)