Advanced, gaining ground initially — hour's duration (6)
I believe the answer is:
length
'duration' is the definition.
(synonyms)
'advanced gaining ground initially hour's' is the wordplay.
'advanced' becomes 'lent' (advance can mean to lend).
'gaining' is an insertion indicator.
'initially' indicates taking the first letters.
'hour' becomes 'h'.
The initial letter of 'ground' is 'g'.
'lent' placed around 'g' is 'lengt'.
'lengt'+'h'='LENGTH'
(Other definitions for length that I've seen before include "Duration or distance" , "Measurement from end to end" , "Something's largest dimension" , "stretch" , "End-to-end distance" .)