Coach begins to fill a gap in niece's muddled knowledge (7)
I believe the answer is:
science
'knowledge' is the definition.
(science is a kind of knowledge)
'coach begins to fill a gap in niece's muddled' is the wordplay.
'begins' says to take the initial letters.
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'fill a gap in' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'muddled' is an anagram indicator.
The initial letter of 'coach' is 'c'.
'niece' is an anagram of 'ience'.
'c' after 's' is 'sc'.
'sc'+'ience'='SCIENCE'
Can you help me to learn more?
(Other definitions for science that I've seen before include "Subject such as biology, physics, etc" , "The search for facts" , "Study of physical things by observation and experiment" , "-- Fiction; -- park" , "a sort of fiction" .)