Lament flash inside head (6)
I believe the answer is:
bemoan
'lament' is the definition.
('bemoan' can be a synonym of 'lament')
'flash inside head' is the wordplay.
'flash' becomes 'mo' (I've seen this in other clues).
'inside' means one lot of letters goes inside another.
'head' becomes 'bean' (bean is a kind of head**).
'mo' going into 'bean' is 'BEMOAN'.
(Other definitions for bemoan that I've seen before include "Deplore, lament" , "Complain about, lament" , "Regret, lament" , "Lament, grieve" .)