A couple of feet? Hoskins is bloody sizable primarily! (5)
I believe the answer is:
'a couple of feet?' is the definition.
(an iamb is a metrical foot in poetry)
'hoskins is bloody sizable primarily' is the wordplay.
'hoskins is' becomes 'I am' (where the crossword setter is 'Hoskins').
'primarily' indicates taking the first letters.
The first letters of 'bloody sizable' is 'bs'.
(Other definitions for iambs that I've seen before include "feet?", "bits of poetry".)