Mo Farah: "Initially, I had led off; that's less important" (6,6)
I believe the answer is:
second fiddle
'that's less important' is the definition.
(as in the phrase 'playing second fiddle')
'mo farah initially i had led off' is the wordplay.
'mo' becomes 'second' (I've seen this before).
'initially' suggests taking the first letters.
'i had' becomes 'id' (common contraction).
'off' indicates an anagram.
The first letter of 'farah' is 'f'.
'led' is an anagram of 'dle'.
'second'+'f'+'id'+'dle'='SECOND FIDDLE'
(Other definitions for second fiddle that I've seen before include "Junior player" , "supporting role" , "should take a bow?" , "junior role" , "In which role Clegg" .)