Excellent and correct, keeping first of them in, letting nothing out (8)
I believe the answer is:
airtight
'letting nothing out' is the definition.
I can't judge whether this defines the answer.
'excellent and correct keeping first of them' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'and' means one lot of letters go next to another.
'correct' becomes 'right' (right can mean correct).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'first of them' becomes 't' (1st letter of 'them').
'right' enclosing 't' is 'rtight'.
'ai'+'rtight'='AIRTIGHT'
'in' is the link.
Can you help me to learn more?
(Other definitions for airtight that I've seen before include "Firmly closed" , "Totally sealed" , "beyond question" , "Impenetrable" , "Not allowing gas to pass" .)