Twenty five pounds of phosphorous about to cover yard (4)
I believe the answer is:
pony
'twenty five pounds' is the definition.
'of phosphorous about to cover yard' is the wordplay.
'of' becomes 'o''.
'phosphorous' becomes 'np' (I am not sure about this - if you are sure you should believe this answer much more).
'about' says the letters should be written backwards.
'to cover' indicates putting letters inside.
'yard' becomes 'y' (abbreviation).
'np' backwards is 'pn'.
'o' put inside 'pn' is 'pon'.
'pon'+'y'='PONY'
Can you help me to learn more?
(Other definitions for pony that I've seen before include "(girl's pet, perhaps)" , "Horse (worth GBP25?)" , "Small breed of horse" , "Arab possibly" , "Horse (worth PS25?)" .)