Feel half-grotty before gym lesson (5)
I believe the answer is:
grope
'feel' is the definition.
(I've seen this before)
'half-grotty before gym lesson' is the wordplay.
'half' means to take half the letters.
'before' means one lot of letters go next to another.
'gym lesson' becomes 'pe'.
'grotty' with half the letters taken is 'gro'.
'gro'+'pe'='GROPE'
(Other definitions for grope that I've seen before include "Paw" , "Fumble" , "Search by hand" , "touch up" , "Feel uncertainly along" .)