Systematic plan army curtailed (7)
I believe the answer is:
regimen
'systematic plan' is the definition.
'regimen' can be an answer for 'plan' (regimen is a kind of plan). I am not certain of the 'systematic' bit.
'army curtailed' is the wordplay.
I cannot quite see how this works, but
'army' could be 're' (I've seen this in other clues) and 're' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for regimen that I've seen before include "prescribed treatment" , "administration" , "Mere gin for a course of treatment" , "System of diet etc" , "Government - course of treatment" .)