Trainspotter losing head — it's like a sieve (5)
I believe the answer is:
'it's like a sieve' is the definition.
(ricers are similar to sieves)
'trainspotter losing head' is the wordplay.
'trainspotter' becomes 'gricer' (gricer can mean a trainspotter).
'losing head' means to remove the first letter.
'gricer' with its initial letter taken away is 'RICER'.
(Other definitions for ricer that I've seen before include "American kitchen utensil" , "Sieve (US)" .)