Snap swell of tide - what are the odds? (6)
I believe the answer is:
selfie
'snap' is the definition.
(type of photograph)
'swell of tide what are the odds?' is the wordplay.
'what are the odds?' means one should take alternating letters (only the odd-numbered letters are used).
'swell'+'of'+'tide'='swelloftide'
The alternating letters of 'swelloftide' are 'SELFIE'.
(Other definitions for selfie that I've seen before include "21 [PHOTO]" , "Personal photo" .)