Number like one on shirt's back (5)
I believe the answer is:
digit
'number' is the definition.
(I know that digit is a type of integer)
'like one on shirt's back' is the wordplay.
'like' becomes 'dig' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'on' says to put letters next to each other.
'back' suggests the final letters (the letter at the back).
The final letter of 'shirt' is 't'.
'dig'+'i'+'t'='DIGIT'
(Other definitions for digit that I've seen before include "Finger; numeral" , "Like It?" , "A small number" , "that's on hand" , "Single figure - finger" .)