Small force blocking river in retreat (5)
I believe the answer is:
elfin
'small' is the definition.
(similar in meaning)
'force blocking river in retreat' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'blocking' indicates putting letters inside (block can mean to plug a gap in something).
'river' becomes 'nile' (Nile is an example).
'in retreat' is a reversal indicator.
'f' going within 'nile' is 'nifle'.
'nifle' reversed gives 'ELFIN'.
(Other definitions for elfin that I've seen before include "Sprightly" , "Small, delicately-framed" , "Pixie-like" , "Mischievous sprite" , "little character" .)