For every system start one way: keep at it (7)
I believe the answer is:
persist
'keep at it' is the definition.
(I've seen this in another clue)
'for every system start one way' is the wordplay.
'for every' becomes 'per' ('per' means 'for each' or 'for every').
'start' suggests taking the first letters.
'one' becomes 'i' (Roman numeral).
'way' becomes 'st' (abbreviation for street).
The first letter of 'system' is 's'.
'per'+'s'+'i'+'st'='PERSIST'
(Other definitions for persist that I've seen before include "Continue to exist" , "Continue firmly or obstinately" , "Endure" , "carry on" , "Persevere, hang on" .)