Private, one lacking energy to keep going (7,2)
I believe the answer is:
soldier on
'keep going' is the definition.
(I've seen this before)
'private one lacking energy' is the wordplay.
'private' becomes 'soldier' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lacking' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mco).
'one' with 'e' taken away is 'on'.
'soldier'+'on'='SOLDIER ON'
'to' is the link.
Can you help me to learn more?
(Other definitions for soldier on that I've seen before include "Persevere (colloq.)" , "Persist in spite of difficulties" , "Keep struggling" , "Doggedly continue" , "... to bear up" .)