One might gush over turning 51 in a positive way (3,4)
I believe the answer is:
oil well
'one might gush over turning 51' is the definition.
I can't judge whether this definition defines the answer.
'a positive way' is the wordplay.
'a positive' becomes 'oil' (I can't explain this - if you can you should give a lot more credence to this answer).
'way' becomes 'well' (well is a kind of way).
'oil'+'well'='OIL-WELL'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for oil well that I've seen before include "Bore for energy source" , "Will Leo? (anag.)" , "Instruction on machine perhaps" , "source of wealth" , "as result of a strike" .)