A type of informal shirt, big and loose (2,5)
I believe the answer is:
at large
'loose' is the definition.
(synonyms)
'a type of informal shirt big' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'shirt' could be 't' (as in a T-shirt) and 't' is found within the answer.
'big' could be 'large' (synonyms) and 'large' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'and' is the link.
Can you help me to learn more?
(Other definitions for at large that I've seen before include "Escaped; as a whole" , "Glare at, being free and not confined" , "Not confined, like escaped prisoner" , "Free, not in custody" , "in a big way!" .)