Push the envelope of danger: Boatman's following (5)
I believe the answer is:
drive
'push' is the definition.
('drive' can be a synonym of 'push')
'the envelope of danger boatman's following' is the wordplay.
'the envelope of' suggests removing the centre (envelope can mean the outside of something).
'boatman's' becomes 'ive' (I can't explain this - if you can you should believe this answer much more).
'following' is a charade indicator (letters next to each other).
'danger' with its centre taken out is 'dr'.
'dr'+'ive'='DRIVE'
(Other definitions for drive that I've seen before include "tenacity" , "gear" , "Motivation, energy" , "Force into action, impel" , "Propel; outing" .)