Come after son, breaking sequence occasionally on reflection (5)
I believe the answer is:
ensue
'come after' is the definition.
(I've seen this before)
'son breaking sequence occasionally on reflection' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'breaking' indicates putting letters inside.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'on reflection' says the letters should be written backwards.
The alternating letters of 'sequence' are 'eune'.
's' going into 'eune' is 'eusne'.
'eusne' back-to-front is 'ENSUE'.
(Other definitions for ensue that I've seen before include "Happen - to come later" , "Transpire" , "Befall" , "Leading directly to" , "Occur afterwards" .)