Game turned round: possibly I initially underestimated Man U (7)
I believe the answer is:
uranium
'u' is the definition.
(synonyms)
'game turned round possibly i initially underestimated man' is the wordplay.
'game' becomes 'ru' (rugby union).
'turned round' shows that the letters should be reversed in order.
'possibly' indicates anagramming the letters.
'initially' indicates taking the first letters.
The initial letter of 'underestimated' is 'u'.
'ru' reversed gives 'ur'.
'i'+'u'+'man'='iuman'
'iuman' with letters rearranged gives 'anium'.
'ur'+'anium'='URANIUM'
(Other definitions for uranium that I've seen before include "Radioactive element used for nuclear energy" , "Radioactive element of atomic number 92" , "Radioactive metallic element" , "Chemical element used to get nuclear energy" , "Radioactive material" .)