Load up with fairly odd leadlights (4)
I believe the answer is:
fill
'load up' is the definition.
'fill' can be an answer for 'load' (thesaurus). I am not sure about the 'up' bit.
'with fairly odd leadlights' is the wordplay.
'with' is a charade indicator (letters next to each other).
'odd' indicates alternate letters.
'leadlights' becomes 'l' (this could be a standard abbreviation I've not previously seen).
The alternate letters of 'fairly' are 'fil'.
'fil' next to 'l' is 'FILL'.
Can you help me to learn more?
(Other definitions for fill that I've seen before include "Be suitable - . . . . the bill" , "Occupy the whole of" , "Plug (a gap)" , "Become satiated" , "Completely occupy" .)