Chief initially recklessly (9)
I believe the answer is:
headfirst
'recklessly' is the definition.
(thesaurus)
'chief initially' is the wordplay.
'chief' becomes 'head' (synonyms).
'initially' becomes 'first' (I've seen this in another clue).
'head'+'first'='HEADFIRST'
(Other definitions for headfirst that I've seen before include "rashly" , "If trashed, dive as divers usually do" , "Without thinking" , "Impetuously" .)