Do it, being a step ahead (7,2)
I believe the answer is:
measure up
'do' is the definition.
(measuring up is a kind of doing)
'step ahead' is the wordplay.
'step' becomes 'measure' (synonyms).
'ahead' becomes 'up' (similar in meaning).
'measure'+'up'='MEASURE UP'
'it being a' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for measure up that I've seen before include "Reach required standard" , "Have what it takes" , "Reach the expected standard" , "take dimensions" , "Pass" .)