Iron put over large and a small jumper (4)
I believe the answer is:
flea
'small jumper' is the definition.
(I've seen this in another clue)
'iron put over large and a' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'put over' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'and' says to put letters next to each other.
'fe' enclosing 'l' is 'fle'.
'fle'+'a'='FLEA'
(Other definitions for flea that I've seen before include "In one's ear, it's a stinging rebuke" , "Wingless, jumping insect" , "Small jumper in small circus" , "Irritating jumper?" , "high-jumper" .)