Letter of alphabet representing measure of tyre inflation (3)
I believe the answer is:
psi
This clue is a double definition.
'letter of alphabet' is the first definition.
(letter of the Greek alphabet)
'measure of tyre inflation' is the second definition.
(PSI is short for pounds per square inch)
'representing' is the link.
(Other definitions for psi that I've seen before include "Greek character" , "foreign letter" , "Twenty-third letter of Greek alphabet" , "Pounds per sq. in. (abbr.)" , "Greek letter; pressure unit" .)