March from base, mostly slow going at first (8)
I believe the answer is:
footslog
'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'.
'foot'+'slo'+'g'='FOOTSLOG'
'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" .)