The 51st catalogue? (4)
I believe the answer is:
list
'catalogue?' is the definition.
(a catalogue is a list of items)
'the 51st' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
The remaining letters 'lis' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for list that I've seen before include "Ordered array of items" , "band > record" , "Itemised catalogue" , "Heel - catalogue" , "Inventory" .)