Letter from Greece with unambiguous description of reactor (7)
I believe the answer is:
nuclear
'description of reactor' is the definition.
'nuclear' can be an answer for 'reactor' (synonyms). I am not certain of the 'description of' bit.
'letter from greece with unambiguous' is the wordplay.
'letter from greece' becomes 'nu' (letter from the Greek alphabet).
'with' means one lot of letters go next to another.
'unambiguous' becomes 'clear' (associated in meaning).
'nu'+'clear'='NUCLEAR'
Can you help me to learn more?
(Other definitions for nuclear that I've seen before include "Using atomic energy" , "source of energy" , "Relating to atomic structure" , "An ulcer (anag)" , "Relating to atomic power" .)