Both sides abandon long aspect when gambling (2,4)
I believe the answer is:
on spec
'gambling' is the definition.
(I've seen this before)
'both sides abandon long aspect' is the wordplay.
'both sides abandon' means to look at the middle letters (the letters on the sides are removed).
'aspect' becomes 'spec' (I am not sure about this - if you are sure you should believe this answer much more).
The centre of 'long' is 'on'.
'on'+'spec'='ON SPEC'
'when' acts as a link.
(Other definitions for on spec that I've seen before include "The sort of hopeful punt" , "gambling" , "Taking a gamble" , "experimentally" .)