Clue: Mushy peas served with pork pie, right for trainer (8)
Answer: ESPALIER
Explanation: Definition: trainer
A charade of ESPA, an anagram ('mushy') of 'peas' plus LIE ('pork pie', rhyming slang) plus R ('right'), with a slightly cryptic definition: an espalier is a lattice on which a plant may be trained.