Condescend to take up soldier in retreat (5)
I believe the answer is:
deign
'condescend' is the definition.
(synonyms)
'up soldier in retreat' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'soldier' becomes 'gi' (GI can mean a US soldier).
'in' indicates putting letters inside.
'retreat' becomes 'den' (both can mean a lair or hideout).
'gi' reversed gives 'ig'.
'ig' placed inside 'den' is 'DEIGN'.
'to take' is the link.
(Other definitions for deign that I've seen before include "Think fit" , "Think appropriate" , "See fit" , "Condescend; stoop" , "consent" .)