Bigwig about to come back for reptile (5)
I believe the answer is:
viper
'for reptile' is the definition.
(viper is a kind of reptile)
'bigwig about to come back' is the wordplay.
'bigwig' becomes 'VIP' (I've seen this in another clue).
'about' becomes 're' ('re' means regarding or about).
'to come back' says the letters should be written backwards.
're' back-to-front is 'er'.
'vip'+'er'='VIPER'
(Other definitions for viper that I've seen before include "Malicious type" , "Malicious person" , "Deadly-venomous snake" , "Venomous snake, person" , "poisonous type" .)