Tetchy self-righteous son occupying shower? (7)
I believe the answer is:
waspish
'tetchy' is the definition.
'self-righteous son occupying shower?' is the wordplay.
'self-righteous' becomes 'pi' (pi is an old-fashioned slang term for pious).
'son' becomes 's'.
'occupying' is an insertion indicator.
'shower?' becomes 'wash' (shower is a kind of wash).
'pi'+'s'='pis'
'pis' put into 'wash' is 'WASPISH'.
(Other definitions for waspish that I've seen before include "Bad-tempered like striped insect" , "'Very irritable, like a stinger (7)'" , "ill-tempered" , "Spiteful (with a slender waist?)" , "Splenetic" .)