Once more put column up to block second test? (9)

'once more put' is the definition.
('deposit' is an answer for 'put' so perhaps 'redeposit' is OK for 'once more put')

'column up to block second test?' is the wordplay.
'column' becomes 'oped' (I can't explain this - if you can you should give a lot more credence to this answer).
'up' is a reversal indicator (in down clue: letters go upwards).
'to block' means one lot of letters goes inside another (block can mean to plug a gap).
'second test?' becomes 'resit' (sitting an exam for a second time).
'oped' back-to-front is 'depo'.
'depo' inserted into 'resit' is 'REDEPOSIT'.

Can you help me to learn more?