Muscle spasm (5)
'muscle spasm' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cramp that I've seen before include "writers may be familiar with" , "restraint" , "Severe muscular pain" , "Confine - muscular spasm" , "Give too little space to" .)
(Other definitions for writers cramp that I've seen before include "Scrivener's palsy" , "Author's hand pain" , "Ailment affecting scribes" .)