Yours truly is bound to begin with a metrical foot (4)
I believe the answer is:
iamb
'metrical foot' is the definition.
(iamb is a kind of metrical foot)
'yours truly is bound to begin with a' is the wordplay.
'yours truly is' becomes 'I'm' (contraction of 'I am').
'to begin' indicates taking the first letters.
'with' is an insertion indicator.
The first letter of 'bound' is 'b'.
'im'+'b'='imb'
'imb' placed around 'a' is 'IAMB'.
(Other definitions for iamb that I've seen before include "Metrical foot of one short, one long syllables" , "Line of verse" , "Short-long metric foot" , "Poet's foot" , "Short-long metrical foot" .)