Feet and inches are measurements British insist on using first of all (5)
I believe the answer is:
iambi
'feet' is the definition.
(plural of iamb, a type of metrical foot in poetry)
'inches are measurements british insist on using first of all' is the wordplay.
I cannot quite understand how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found within the answer.
'british' could be 'b' (abbreviation e.g. in 'BBC') and 'b' is found in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is found within the answer.
The remaining letters 'mi' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'and' is the link.
Can you help me to learn more?
(Other definitions for iambi that I've seen before include "Concierto de Aranjuez composer" , "Poetic feet" , "Feet of two syllables (one short and one long)" , "Film theatre" , "Short-long metrical feet" .)