Soldiers having drinks with recreational drug get ill again (7)
I believe the answer is:
relapse
'get ill again' is the definition.
'soldiers having drinks with recreational drug' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'having' says to put letters next to each other.
'drinks' becomes 'laps' (lapping is a kind of drinking**).
'with' means one lot of letters go next to another.
'recreational drug' becomes 'e' (E can refer to ecstasy).
're'+'laps'+'e'='RELAPSE'
(Other definitions for relapse that I've seen before include "falling back" , "Get into bad habits again" , "setback in health terms" , "Degenerate like pleaser" , "Fall back (into ill health)" .)