In being expansive, a chap is out of order! (5)
I believe the answer is:
roomy
'in being expansive a' is the definition.
I can't judge whether this defines the answer.
'chap is out of order' is the wordplay.
'chap' becomes 'romy' (I can't justify this - if you can you should believe this answer much more).
'is out of' means one lot of letters goes inside another (some letters on the outside of others).
'order' becomes 'o' (abbreviation e.g. in British honours OM, OBE etc.).
'romy' going around 'o' is 'ROOMY'.
Can you help me to learn more?
(Other definitions for roomy that I've seen before include "Broad" , "Comfortably large" , "With ample space" , "Spaced-out" , "Having big space" .)