Study always covers troublesome Horace account (11)
I believe the answer is:
archaeology
'study' is the definition.
(archaeology is a kind of study)
'always covers troublesome horace account' is the wordplay.
'always' becomes 'ay'.
'covers' means one lot of letters goes inside another.
'troublesome' indicates anagramming the letters.
'account' becomes 'log'.
'horace' anagrammed gives 'rchaeo'.
'rchaeo'+'log'='rchaeolog'
'ay' going around 'rchaeolog' is 'ARCHAEOLOGY'.
(Other definitions for archaeology that I've seen before include "Study of old remains" , "Study of history through remains" .)