Truss very upset after a fitting (7)
I believe the answer is:
apropos
'fitting' is the definition.
(I've seen this before)
'truss very upset after a' is the wordplay.
'truss' becomes 'prop' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'very' becomes 'so' (both can mean 'extremely').
'upset' says the letters should be written in reverse.
'after' says to put letters next to each other.
'so' reversed gives 'os'.
'prop'+'os'='propos'
'propos' after 'a' is 'APROPOS'.
Can you help me to learn more?
(Other definitions for apropos that I've seen before include "With regard to" , "Appropriate" , "In respect of" , "On the subject of" , "Suitable; to the point" .)