Barking athlete initially managed to maintain fast time (7)
I believe the answer is:
ramadan
'fast time' is the definition.
(month in which Muslims fast)
'barking athlete initially managed to maintain' is the wordplay.
'barking' becomes 'mad' (barking mad).
'initially' indicates taking the first letters.
'managed' becomes 'ran' (I've seen this before).
'to maintain' is an insertion indicator (maintain can mean to keep or preserve).
The initial letter of 'athlete' is 'a'.
'mad'+'a'='mada'
'mada' put inside 'ran' is 'RAMADAN'.
(Other definitions for ramadan that I've seen before include "Ninth month of Muslim year, a time of fasting" , "Ninth month of Muslim year, time to fast" , "The fasting month for Muslims, ninth of year" , "Islam holy month" , "lunar month" .)