Rather old hat but popular with the boys (5)
I believe the answer is:
dated
'rather old hat' is the definition.
'dated' can be an answer for 'hat' (I have seen 'Old hat' mean 'dated' so perhaps 'hat' could also mean 'dated'). I'm not certain of the 'rather old' bit.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for dated that I've seen before include "Rusty" , "Out of fashion" , "square" , "Revealed the age of" , "Walked out with" .)