Before maiden, first slip joins extra cover (7)
I believe the answer is:
smother
'cover' is the definition.
(smothering is a kind of covering)
'before maiden first slip joins extra' is the wordplay.
'before' means one lot of letters go next to another.
'maiden' becomes 'm' (cricket abbreviation).
'first slip' becomes 's' (ost letter of 'slip').
'joins extra' becomes 'other' (I am not sure about this - if you are sure you should believe this answer much more).
'm' after 's' is 'sm'.
'sm'+'other'='SMOTHER'
Can you help me to learn more?
(Other definitions for smother that I've seen before include "Kill by depriving of air" , "blanket" , "Extinguish (fire) by covering it" , "don't give enough freedom" , "Conceal" .)