Fifty-one feet of lead flashing required for launch (4-3)
I believe the answer is:
lift-off
'launch' is the definition.
(I've seen this before)
'fifty-one feet of lead flashing required' is the wordplay.
'fifty one' becomes 'LI' (Roman numerals).
'of lead' indicates taking the first letters (I've seen 'leads' mean this).
'flashing' becomes 'toff' (I can't justify this - if you can you should give a lot more credence to this answer).
'required' means one lot of letters go next to another.
The initial letter of 'feet' is 'f'.
'li'+'f'+'toff'='LIFT-OFF'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for lift-off that I've seen before include "Instant when rocket leaves launching pad" , "Launch (of rocket)" , "Start of rocket's journey" , "Moment of rocket's launch" .)