A Scottish singer, laid back, oddly trendy, wailed (8)

'wailed' is the definition.
('ululate' can be a synonym of 'wail')

'a scottish singer laid back oddly trendy' is the wordplay.
'scottish singer' becomes 'lulu' ().
'laid back' is a reversal indicator.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternating letters of 'trendy' are 'ted'.
'a'+'lulu'='alulu'
'alulu' in reverse letter order is 'ulula'.
'ulula'+'ted'='ULULATED'