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 see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
The remaining letters 'lis' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
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" .)