Attempt (no turning back) to test for size and suitability (3,2)
I believe the answer is:
try on
'attempt' is the definition.
(I know that try-on is a type of run)
'no turning back to test for size and suitability' is the wordplay.
I cannot really see how this works, but
'no' could be 'o' (O resembles 0) and 'o' is located in the answer.
'test' could be 'try' (synonyms) and 'try' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for try on that I've seen before include "Attempt at deception" , "attempt to deceive" , "Check the fit of (a garment)" , "audacious imposition" , "attempted deception" .)