More meagre, containing bank's last large diamond (8)
I believe the answer is:
sparkler
'diamond' is the definition.
(something which sparkles)
'more meagre containing bank's last large' is the wordplay.
'more meagre' becomes 'sparer' ('spare' can be similar in meaning to 'meagre').
'containing' is an insertion indicator.
'last' says to take the final letters.
'large' becomes 'L' (eg in clothes sizes).
The final letter of 'bank' is 'k'.
'k'+'l'='kl'
'sparer' going around 'kl' is 'SPARKLER'.
(Other definitions for sparkler that I've seen before include "Firework - precious stone" , "Hand-held firework" , "Bright gem" , "Jewel" , "Firework or diamond ring" .)