Old songs left for silly people mostly to collect (5)

'old songs' is the definition.
'glees' can be an answer for 'songs'. I'm unsure of the 'old' bit.

'left for silly people mostly to collect' is the wordplay.
'left for' becomes 'l' (I can't explain this - if you can you should believe this answer much more).
'silly people' becomes 'geese' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'to collect' indicates putting letters inside (inserted letters are collected by others).
'geese' with its last letter removed is 'gees'.
'l' going inside 'gees' is 'GLEES'.

Can you help me to learn more?