Don't fall behind as stronghold's erected (4,2)
I believe the answer is:
keep up
'don't fall behind as' is the definition.
I can't tell whether this definition defines the answer.
'stronghold's erected' is the wordplay.
'stronghold' becomes 'keep' (keep is a kind of stronghold).
'erected' becomes 'up'.
'keep'+'up'='KEEP UP'
Can you help me to learn more?
(Other definitions for keep up that I've seen before include "Continue to be in touch (with)" , "Maintain pace with - the Jones?" , "' - - with the Joneses'" , "Stay with the pace, don't fall behind" , "Stay in contention" .)