Friend follows soldiers, in truth (6)
I believe the answer is:
really
'in truth' is the definition.
('really' can be a synonym of 'in truth')
'friend follows soldiers' is the wordplay.
'friend' becomes 'ally' (I've seen this before).
'follows' says to put letters next to each other.
'soldiers' becomes 're' (Royal Engineers**).
'ally' after 're' is 'REALLY'.
(Other definitions for really that I've seen before include "Genuinely" , "without a doubt" , "Actually, in fact" , "In genuine way" , "Actually, no kidding" .)