Victor and Oscar have whiskey on the sierra and nuns have to take them (4)
I believe the answer is:
vows
'nuns have to take them' is the definition.
I can't tell whether this defines the answer.
'victor and oscar have whiskey on the sierra' is the wordplay.
'victor' becomes 'V' (phonetic alphabet: alpha, bravo, charlie etc.).
'and' means one lot of letters go next to another.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'have whiskey on' becomes 'w' (I can't justify this - if you can you should give a lot more credence to this answer).
'the sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'v'+'o'+'w'+'s'='VOWS'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for vows that I've seen before include "Pledges or religious oaths" , "Solemn promises" , "Oaths and promises" .)