Optimistic Bishop invested in universal fuel source (6)
I believe the answer is:
upbeat
'optimistic' is the definition.
(similar in meaning)
'bishop invested in universal fuel source' is the wordplay.
'bishop' becomes 'B' (abbreviation for bishop in chess).
'invested in' indicates putting letters inside.
'universal' becomes 'U' (film certificate).
'fuel source' becomes 'peat' (I've seen this in another clue).
'u'+'peat'='upeat'
'b' put inside 'upeat' is 'UPBEAT'.
(Other definitions for upbeat that I've seen before include "Of mood, cheerful and optimistic" , "Promising" , "signal from sergeant, say, at end of bar?" , "Cheerfully optimistic" , "Optimistic (colloq.)" .)