Iron increases with these exercises (5-3)
I believe the answer is:
press-ups
'these exercises' is the definition.
(press-up is a kind of exercise)
'iron increases' is the wordplay.
'iron' becomes 'press' (I've seen this before).
'increases' becomes 'ups' (upping is a kind of increasing).
'press'+'ups'='PRESS-UPS'
'with' acts as a link.
(Other definitions for press-ups that I've seen before include "Exercises performed face down" , "Exercises done facing the floor" .)