Seven years at the coalface — finally moan constantly, effective at first (7)
I believe the answer is:
teenage
'coalface finally moan constantly effective at first' is the definition.
I can't judge whether this defines the answer.
'seven years at the' is the wordplay.
'seven' becomes 'een' (I can't justify this - if you can you should believe this answer much more).
'years' becomes 'age' (period of many years).
'at' says to put letters next to each other (I've seen this in other clues).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'een'+'age'='eenage'
'eenage' after 't' is 'TEENAGE'.
Can you help me to learn more?
(Other definitions for teenage that I've seen before include "of seven difficult years" , "Juvenile" , "Relatively young" , "Gee, neat (anag)" , "Like one thirteen-nineteen years old" .)