Staunch son encountered going north (4)
I believe the answer is:
stem
'staunch' is the definition.
(I know that staunch can be written as stem)
'son encountered going north' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'going north' says the letters should be written in reverse.
'met' back-to-front is 'tem'.
's'+'tem'='STEM'
(Other definitions for stem that I've seen before include "Central upright part of flower or drinking glass" , "Upward part of a plant" , "Flower's stick" , "section of pipe" , "Staunch (flow)" .)