Stresses seem periodically to accompany developmental stages (8)
I believe the answer is:
emphases
'stresses' is the definition.
('emphasis' can be a synonym of 'stress')
'seem periodically to accompany developmental stages' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'to accompany' says to put letters next to each other.
'developmental stages' becomes 'phases' (I've seen this in another clue).
The alternate letters of 'seem' are 'em'.
'em'+'phases'='EMPHASES'
(Another definition for emphases that I've seen is " Stresses".)