According to relative, coverage is constant (10)
I believe the answer is:
persistent
'is constant' is the definition.
(thesaurus)
'according to relative coverage' is the wordplay.
'according to' becomes 'per' (I've seen this in another clue).
'relative' becomes 'sis' (short for sister).
'coverage' becomes 'tent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'per'+'sis'+'tent'='PERSISTENT'
Can you help me to learn more?
(Other definitions for persistent that I've seen before include "Tenacious; constant" , "single-minded" , "Lasting, prolonged" , "Ongoing" , "Dogged" .)