Strong face lacks depth but shows some promise (9)
I believe the answer is:
potential
'some promise' is the definition.
(I've seen this before)
'strong face lacks depth' is the wordplay.
'strong' becomes 'potent' (synonyms).
'face' becomes 'dial' (face of a clock).
'lacks' suggests deleting specific letters.
'depth' becomes 'd' (abbreviation in geometry).
'dial' with 'd' taken out is 'ial'.
'potent'+'ial'='POTENTIAL'
'but shows' acts as a link.
(Other definitions for potential that I've seen before include "Inherent capacity not realised" , "Possibility, promise" , "Possible (capacities of a person or situation)" , "Latent qualities that may develop" , "Powerful" .)