Confession of giant lacking good feet (5)
I believe the answer is:
iambi
'feet' is the definition.
(plural of iamb, a type of metrical foot in poetry)
'confession of giant lacking good' is the wordplay.
'confession of' becomes 'iam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'giant' becomes 'big' (similar in meaning).
'lacking' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'big' with 'g' taken away is 'bi'.
'iam'+'bi'='IAMBI'
(Other definitions for iambi that I've seen before include "Poetic pieces" , "Metrical patterns" , "Film theatre" , "Concierto de Aranjuez composer" , "Feet of two syllables (one short and one long)" .)