March from base, mostly slow going at first (8)
I believe the answer is:
'march' is the definition.
(I've seen this before)
'base mostly slow going at first' is the wordplay.
'base' becomes 'foot' ('foot' can be a synonym of 'base'**).
'mostly' means to remove the last letter.
'at first' suggests taking the first letters.
The first letter of 'going' is 'g'.
'slow' with its last letter taken off is 'slo'.
'from' acts as a link.
(Other definitions for footslog that I've seen before include "Long exhausting walk" , "what soldier does" , "exhausting hike" , "arduous journey" , "Tramp" .)