Record thanks at start of gym class (4)
I believe the answer is:
tape
'record' is the definition.
(synonyms)
'thanks at start of gym class' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'at start of' says to put letters next to each other (some letters go in front of others).
'gym class' becomes 'pe' (abbreviation for physical education).
'ta'+'pe'='TAPE'
(Other definitions for tape that I've seen before include "The winning runner breasts the ...." , "Adhesive --; red --" , "Common medium for recording" , "broken by fastest runner" , "Strip of material (that might be red!)" .)