Tend to like such a formal shirt perhaps as party wear? (5,5)
I believe the answer is:
fancy dress
'wear?' is the definition.
(fancy dress is a kind of wear)
'tend to like such a formal shirt perhaps as party' is the wordplay.
I cannot quite understand how this works, but
'like' could be 'y' (similar in meaning) and 'y' is found in the answer.
'a' could be 'an' and 'an' is found within the answer.
'formal' could be 'dress' (similar in meaning) and 'dress' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for fancy dress that I've seen before include "Masquerade costume?" , "Party costume" , "Frivolous party garb" , "Themed party attire" .)