His pa is somehow simian (5)
I believe the answer is:
apish
'simian' is the definition.
(I've seen this in another clue)
'his pa is somehow' is the wordplay.
'is somehow' is an anagram indicator.
'his'+'pa'='hispa'
'hispa' with letters rearranged gives 'APISH'.
(Other definitions for apish that I've seen before include "Foppish" , "Foolish; like a monkey" , "Silly" , "Stupid" , "Foolish; simian" .)