Dominant peak on Hawaii (5)
I believe the answer is:
alpha
'dominant' is the definition.
(I've seen this before)
'peak on hawaii' is the wordplay.
'peak' becomes 'alp ' (I've seen this before).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'hawaii' becomes 'HA' (abbreviation for Hawaii).
'alp'+'ha'='ALPHA'
(Other definitions for alpha that I've seen before include "starter" , "Top grade" , "First-rate" , "Greek vowel" , "star of utmost brilliance" .)