An average sort of shirt is indicated (5)
I believe the answer is:
meant
'is indicated' is the definition.
(I've seen this in another clue)
'an average sort of shirt' is the wordplay.
'an average' becomes 'mean' (type of average).
'sort of shirt' becomes 't' (I've seen this in other clues).
'mean'+'t'='MEANT'
(Other definitions for meant that I've seen before include "Intentional" , "represented" , "Had as purpose or design" , "no accident" , "Intended, had in mind" .)